Untitled

Description

Flashcards on Untitled, created by ggreetham on 06/05/2013.
ggreetham
Flashcards by ggreetham, updated more than 1 year ago
ggreetham
Created by ggreetham almost 11 years ago
598
0

Resource summary

Question Answer
Flash memory is nonvolatile True
A sound consists of a waveform with amplitude (volume) and ____ (pitch). Frequency
There are seven levels of RAID True
____ are small, special-purpose devices or computers used to connect two or more networks. Routers
The computer samples sounds at fixed intervals and each sample is assigned a binary value according to its ____. Amplitude
The mouse serves as a primary input device True
CRT displays are much thinner and run at a much cooler temperature than LCD monitors False
A ____ is a device designed to protect an internal network or node from intentional or unintentional damage from an external network Firewall
Which of the following is an example of a video compression format? MPEG
____ convert binary digits into sounds by modulating or modifying a tone Modems
Interrupt handling is a much more efficient method for processing I/0 than polling True
By switching on and off, the ____ can be used to represent the 1s and 0s that are the foundation of all that goes on in the computer Transitor
Normal copper wire is capable of carrying ____ voice channels 24
A digital transmission link with a capacity of 1.544 Mbps is known as a ____ line T1
____ algebra is concerned with the logic of the operators AND, OR, and NOT Boolean
Normal telephone voice-grade lines use ____-division multiplexing to divide up the bandwidth among the subscribers. Frequency
The ____ operator works with a single input, and its purpose is to reverse the input NOT
Because of attenuation, a DSL subscriber is required to be no more than ____ feet away from the nearest telephone company switching station. 18,000
A transistor-based circuit in the computer that implements Boolean logic by creating a single output value for a given set of input values is known as a ____ Gate
Cable modems are capable of speeds up to ____. 42Mbps
The ____ gates are the basic building blocks of the CPU AND, OR and NOT
The NAND gate takes the output of the AND gate and then ____ it with the NOT gate Reverses
The truth table for the ____ gate indicates that the output is 1 only when the inputs are different XOR
A gate’s output for any set of inputs follows the specifications given in the ____ table Truth
One of the main functions of the ____ of the computer’s CPU is to add numbers. ALU
The high-speed cache memory in your computer is made of many thousands of ____ circuits Flip-flop
Vacuum tubes were replaced with ____ Transitors
The MCP is the software “control center” of modern computer systems. False
Without a database most applications would be useless True
A database management system (DBMS) provides an end-user interface with the database True
The operating system is the first program loaded into memory when the computer starts (boots). True
The operating system is the first program loaded into memory when the computer starts (boots). True
Very Large-Scale Integration (VLSI) chips contain ____ of circuits. Millions
When power is turned on, the CPU begins executing the instructions stored in the CMOS chip. False
A DBMS delegates the task of ensuring data integrity to the operating system False
An OS can reside on a ROM chip. True
Applications cannot be designed to run on multiple platforms. False
Most computers today are still based on the ____ architecture Von Neumann
A DBMS manages multiple users’ access to the database True
Clients are computers that are used to service many different users. False
An attribute is also called a relation False
Every multipurpose device with a CPU must also have an OS. True
A collection of columns referring to one item is called a row or tuple True
DOS and Linux use a graphical user interface. False
The portion of the CPU responsible for mathematical operations is the ____ Arithmetic Logic Unit
Indexes do not require additional storage space in the database False
The Windows operating system no longer provides a command-line interface. False
The information stored in a database is kept in natural or sequential order. True
In Windows you can view running processes by pressing Ctrl+Alt+Delete to open Task Manager and then clicking the Processes tab to display information on all running processes. True
From a database design point of view, first normal form is better than second normal form False
Von Neumann architecture supports the parallel execution of instructions. False
One of the basics of Von Neumann architecture is the ____ cycle Fetch-Execute
You could define a database table based on unnormalized tables, but redundancies and dependencies can cause problems when the table is in use. True
The parts of a computer system, such as the CPU, memory, keyboard, monitor, and network adapter, function at different speeds. True
I/O devices will always work correctly from the basic installation in the environment in which they’re installed. False
The term “____” refers to the family of components used to store programs and dat Storage
A determinant is also called a composite key False
A column is functionally dependent on another column if for each value of the first column, there is only one value for the second column True
Plug and Play was intended to create a computer with hardware and software that worked together to configure devices and assign resources automatically, making hardware changes and additions easier. True
The third normal form creates columns that are independent of the primary key False
Deadlock issues are always resolved without the intervention of the operating system. True
Operating systems are responsible for protecting memory and other resources. True
The main rules in database design are the second and third normal forms False
Files can be viewed as leaves (nodes) on a tree structure. True
A one-to-many relationship is the most common relationship True
One-to-one relationships are very common False
The RETURN BY clause enables you to change how data is returned from the SELECT statement False
RAM is called “____” because it doesn’t have to be read sequentiall Random
There are many SQL commands for maintaining, defining, and administering the data in a database True
SQL deals with only the data you want to see. True
SQL commands may not be lowercase False
Every column must be initialized with data False
Hard disks are made up of one or more metal platters with a coating consisting of ____ particles Magnetic
The order of the fields in the SELECT statement doesn’t have to match that of the defined table structure True
The MORE clause in a SQL SELECT statement specifies additional criteria for retrieving data from a table False
In relation to refining searches, the keyword AND indicates that all criteria must be met True
By default, the SELECT statement returns data in no particular order False
Data that has been organized and logically related to allow access, retrieval, and use of that data is called a ____ Database
A special language known as ____ is used to query or extract information from databases Structured Query Language
____ was one of the computer scientists at IBM creating a theoretical model for designing data structures E.F. Codd
You can practice DOS commands by opening a DOS command window in Windows. False
A DBMS helps manage data and extract information from a database by using a(n) ____ language. Query
Linux commands have what are called command-line switches (sometimes called parameters or flags), used in the format: ls -lrt True
A 120-mm disc used to store data, music, and video in a computer system by using laser technology is known as a(n) ___ CD-ROM
A ____ or entity is arranged into rows or columns much like a spreadsheet Table
The set of possible values for each column is called the ____ of that column Domain
eyboards, and most other I/O devices, connect to the main board through a(n) ____ Port
A(n) ____ is a special type of file that occupies its own space and specifies one or more columns that determine how information stored in a table can be accessed more efficiently Index
Windows and Linux use the fdisk command to partition hard drives. True
When the columns are used to determine the sort order of information, they are called sort ____. Keys
Formatting can be used to place a copy of the OS on the disk so that it can be used as a boot disk. True
You cannot create child folders inside user-created folders. False
The parent-child structure has nearly unlimited depth. True
The set of rules that dictates how databases are designed is called ____ Normalization
In general, folder names must start with a letter and can include additional numbers or letters and some special characters, up to a maximum length of 255 characters. True
The process of painting an image on the screen of a monitor is called ____ scanning Raster
To put the table into the ____ normal form, any columns containing two values need to be separated into two rows. First
Command-line OSs such as Linux allow recovering folders and files after they have been deleted. False
Command-line OSs such as Linux allow recovering folders and files after they have been deleted. False
Interface programs that allow a computer to interact with peripheral devices are called ____. Drivers
Printer ____ ranges from 300 dpi to 2400 dpi for both inkjet and laser printers Resolution
The ____ is the core of the OS. Kernel
The OS running on a computer is often called its ____ and is typically tied to a particular CPU. Platform
A ____ key is simply a column or combination of columns that uniquely identifies a row within a table. Primary
Coordinated execution of a process, using two or more CPUs at the same time is known as ____. Multiprocessing
A ____ operating system is an OS designed to provide strong network services. Network
____ is a primary function of an operating system. Managing Resources
The sound card fits into the ____ bus expansion slot on the main board PCI
A(n) ____ key is a primary key made up of more than one column. Composite
DOS and Linux use a(n) ____-line interface and are sometimes called console operating systems. Command
A small program running on a computer is known as a ____. Process
____ was designed to accommodate multiple users and multiple tasks. Linux
Time ____ is a method of allocating fixed time units to running processes so that it appears to users that all processes are running simultaneously. Slicing
Placing a table into second normal form eliminates ____ of data. Repetition
When an interrupt occurs, the CPU stops what it’s doing and goes to a program in memory called a(n) ____ handler. Interrupt
When an interrupt occurs, the CPU stops what it’s doing and goes to a program in memory called a(n) ____ handler. Interrupt
A rare situation in which processes are waiting for each other for use of resources is known as ____. Deadlock
A(n) ____ dependency exists when one column is dependent on another column that isn’t the primary key Transitive
All programs and processes running on a computer are stored in ____. RAM
When an I/O device places a voltage signal on a(n) ____ line, the associated chip checks its priority before passing it on to the CPU Interrupt
A(n) ____ defines how one table (entity) works with or relates to another Relationship
OSs allow you to organize files into structures called folders or ____. Directories
A(n) ____ key is a column in one table that relates to the primary key in another. Foreign
____ are an area of a hard disk reserved to hold files of a particular OS type. Partitions
A visual representation of how all the tables or entities interact and relate to each other in a database is called a(n) ____ relationship model. Entity
In Linux, entering ____command gives you the manual pages with information on the command you specify. man
Factors such as memory type, bus speed, and even hard drive speed can affect overall speed far more than the ____ clock. CPU
In Linus, to exit manual pages, simply type “____.” q
____ shows the numeric occurrences between entities in an ER model Cardinality
The process of ____ arranges the disk surface into addressable areas and sets up the basic directory tree structure on the disk. Formatting
The ____ command enables Linux to partition a disk drive. fdisk
Which of following is a valid SQL command for constructing a table? CREATE TABLE
In Windows ____, you can browse the tree structures of your drives. Explorer
In Windows, deleted files and folders reside in the ____ Bin folder until it is emptied. Recycle
The keyword ____ indicates that data is not required for a specified column NULL
____ are symbols that are used to match any character and are used in selecting files or directories. Wildcards
Wildcard specifications are case sensitive in ____. Linux
SQL statements are closed with a(n) ____. Semicolon
Partitioning, ____, and creating folders are done to prepare the drive to receive files. Formatting
Moving files is similar to ____ files. Copying
The ____ statement is used to add new rows of data to a table. INSERT INTO
The ____ statement is the most commonly used SQL statement SELECT
B2C e-commerce is a useful tool for connecting business partners in a virtual supply chain to cut resupply times and reduce costs. False
Producers or providers of consumer products using B2C e-commerce must use middlemen, or intermediaries, between them and the consumer. False
If you want to arrange the data more meaningfully when using the SELECT statement, you can specify the ____ column names. ORDER BY
G2B applications are designed to improve communications among the various levels of government. False
If you want data returned in descending order, you simply place the keyword ____ next to the column in the ORDER BY clause DESC
E-commerce relies on the use of wireless devices, such as personal digital assistants, cell phones, and smartphones, to place orders and conduct business. False
Each e-commerce Web site must have Web server software to perform fundamental services, including security and identification, retrieval and sending of Web pages, Web site tracking, Web site development, and Web-page development. True
A server that handles dynamic content must be able to access information from a variety of databases. True
The use of open database connectivity enables the Web server to assemble information from different database management systems, such as SQL Server, Oracle, and Informix. True
Product configuration software tools were originally developed in the 1960s to assist B2C salespeople to match their company’s products to customer needs. False
E-commerce software must support five core tasks: catalog management, product configuration, shopping cart facilities, e-commerce transaction processing, and Web traffic data analysis. True
For m-commerce to work effectively, the interface between the wireless device and its user must improve to the point that it is nearly as easy to purchase an item on a wireless device as it is to purchase it on a PC. True
A user with a WAP-compliant device uses the built-in microbrowser to make a WML request. True
A CA is responsible for guaranteeing that the people or organizations that are granted unique digital certificates are, in fact, who they claim to be. True
The DSL communications protocol includes a handshake stage, which authenticates the server (and the client, if needed), determines the encryption and hashing algorithms to be used, and exchanges encryption keys. False
Credit cards are better protected from misuse than conventional smart, charge, and debit cards because the credit-card information is encrypted. False
A smart card makes credit theft practically impossible because a key to unlock the encrypted information is required, and there is no external number that a thief can identify and no physical signature a thief can forge. True
Enterprise systems employ a database of key operational and planning data that can be shared by all. True
Web services can combine software and services from different companies to provide an integrated way to communicate. True
A characteristic of an online transaction processing system is that there is some delay between an event and the eventual processing of the related transaction to update the organization’s records. False
Often, results from one TPS flow downstream to become input to other systems. True
Systems design attempts to answer the questions “What is the problem, and is it worth solving?” False
Systems investigation attempts to answer the question “What must the information system do to solve the problem?” False
Systems design seeks to answer the question “How will the information system do what it must do to obtain the problem solution?” True
Systems analysis details system outputs, inputs, and user interfaces; specifies hardware, software, database, telecommunications, personnel, and procedure components; and shows how these components are related. False
Systems design results in an installed, operational information system that meets the business needs for which it was developed. False
Prototyping begins with creating a preliminary model of a major subsystem or a scaled- down version of the entire system. True
Agile development requires frequent face-to-face meetings with the systems developers and users as they modify, refine, and test how the system meets users’ needs and what its capabilities are. True
The signal-to-noise ratio measures the quality of a communications channel True
XP uses pairs of programmers who work together to design, test, and code parts of the systems they develop. True
JAD often uses group support systems (GSSs) software to foster positive group interactions, while suppressing negative group behavior. True
The bit error rate usually increases as the transfer rate decreases False
Legal feasibility can include logistical and motivational (acceptance of change) considerations. False
The most common guided medium has been silicon False
Legal feasibility involves an analysis of existing and future laws to determine the likelihood of legal action against the systems development project and the possible consequences. True
Every computer game, program, picture, or sound is stored in the computer as a series of decimal digits False
The object-oriented approach can be used during all phases of systems development, from investigation to maintenance and review. True
Unguided media cannot transmit or carry infrared light signals False
Negative powers are used to represent the fractional portion of numbers True
The primary outcome of systems analysis is a prioritized list of systems requirements. True
Data modeling is most often accomplished through the use of entity-relationship (ER) diagrams. True
Activity modeling is often accomplished through the use of data-flow diagrams. True
The hexadecimal numbering system has seventeen unique digits False
DFDs work on the premise that every activity involves some communication, transference, or flow that can be described as a data element. True
Impedance makes the electrical signals weaken as they travel along the wire True
The primary result of the systems maintenance phase is a technical design that details system outputs, inputs, and user interfaces; specifies hardware, software, databases, telecommunications, personnel, and procedures; and shows how these components are related. False
A sequence of events is often called an interface, and can be diagrammed in a sequence diagram. False
Twisted pair cable has been a popular medium in the past, but it’s being replaced in most instances by coaxial cables that are less expensive to produce and have even higher bandwidths False
Parallel start-up involves running the new system for one group of users rather than all users. False
Inductance doesn’t apply to fiber-optic cables at all True
The hexadecimal numbering system uses letters to represent digits beyond nine True
Pilot start-up involves running both the old and new systems for a period of time. False
The term radix is synonymous with positional value. False
A ____ is a professional who specializes in analyzing and designing business systems. Systems analyst
Fiber-optic cables are much more susceptible to attenuation than copper cables False
For any quantity, there’s a number in any base to represent it. True
In systems development, the ____ is responsible for modifying or developing programs to satisfy user requirements. Programmer
A counting table can be used to convert numbers from one base to another. True
In the ____ phase of the SDLC, potential problems and opportunities are identified and considered in light of the goals of the business. Systems Investigation
Wireless networking uses the same technology as the radio in your car and the cell phone in your pocke True
The ____ phase of the SDLC involves studying existing systems and work processes to identify strengths, weaknesses, and opportunities for improvement. Systems analysis
Without protocols, computers would not function True
The primary result of the ____ phase is a technical design that either describes the new system or describes how existing systems will be modified. Systems design
Adding numbers in other bases follows nearly the same steps as in decimal numbers. True
The difference between adding numbers in base 10 and any other base lies in the carry process. True
Without protocols, there would be no Internet True
____ involves creating or acquiring the various system components detailed in the systems design, assembling them, and placing the new or modified system into operation. Systems Implantation
You cannot multiply or divide any base. False
____ takes an iterative approach to the systems development process. During each iteration, requirements and alternative solutions to the problem are identified and analyzed, new solutions are designed, and a portion of the system is implemented. Prototyping
The Data Link layer of the OSI model uses routing protocols to select optimal paths between two end system False
____ employs tools, techniques, and methodologies designed to speed application development. Rapid Application Assessment
Half of a byte is sometimes referred to as a nibble True
RAD makes extensive use of the ____ process for data collection and requirements analysis. Joint application development
Layers in the OSI model are defined and designed to provide services for the process of communicating between computers. True
Computer scientists sometimes use hexadecimal as a shorthand method for representing binary values True
____ combine(s) the logic of the systems development life cycle with the power of object-oriented modeling and programming. Object-oriented systems development
The distinction between LANs and WANs is clear and well defined False
Whole numbers, or integer numbers, can be represented internally in a computer as hexadecimal numbers. False
The computers attached to a network are often referred to as nodes True
Signed and unsigned numbers in a computer are designed to always take up a different number of bits. False
The ____ is a document that is filled out by someone who wants the IS department to initiate systems investigation. Systems request form
There are six basic LAN topologies False
Negative fractional numbers cannot be internally represented in a computer. False
____ feasibility is concerned with whether the hardware, software, and other system components can be acquired or developed to solve the problem. Technical
The Unicode character standard is compatible with the extended ASCII standard. True
____ feasibility determines whether the project makes financial sense and whether predicted benefits offset the cost and time needed to obtain them. Economic
Fast Ethernet speeds do not exceed 10 Mbps. False
____ feasibility determines whether laws or regulations can prevent or limit a systems development project. Legal
____ feasibility is a measure of whether the project can be put into action or operation. Operational
The Unicode method of character representation is fast becoming obsolete. False
The ____ report summarizes the results of systems investigation and the process of feasibility analysis and recommends a course of action: continue on into systems analysis, modify the project in some manner, or drop it. Systems investigation
Each physical device connected to a network must have a network interface card (NIC). True
The number of pixels in each row and column defines a display device’s resolution. True
Repeaters alter the content of the data in the signals that they boost False
A common resolution on a PC monitor is 1024 x 768, which means the monitor is displaying 1024 rows with 768 dots, or pixels, in each row. False
Gateways can be used to connect networks of different types or to connect mainframe computers to PCs True
With ____, one or more members of the analysis team directly observe the existing system in action. Direct Observation
As the resolution numbers get larger for a monitor, the size of each pixel gets larger. False
When many data sources are spread over a wide geographic area, ____ might be the best approach to data collection. Questionnaires
Shifting the starting point of an audio waveform is known as frequency modulatio False
Sounds are stored in the computer in a manner very similar to images. True
____ is the manipulation of the collected data so that the development team members who are participating in systems analysis can use the data. Data analysis
You can compress information by replacing repeating patterns with a code True
The normal copper wire used in your home is capable is actually capable of speeds faster than to 1.5 Mbps True
The number of words used for each sound sample determines how many unique amplitude levels can be represented. False
____ is a commonly accepted approach to modeling organizational objects and associations that employ both text and graphics. Data Modeling
Raw audio samples are often stored in uncompressed file formats, such as WAV files. True
DSL (digital subscriber line) technologies do not allow the upload speed to differ from the download speed False
A(n) ____ models objects, associations, and activities by describing how data can flow between and around various objects. Data flow diagram
The purpose of ____ is to answer the question “How will the information system solve a problem?” Systems design
Cable modems provide for differing upload and download speed True
It is not possible to learn to program or to manage a database without understanding binary and hexadecimal number systems. False
Displays containing the contents of a computer’s memory or the hard disk are often referred to as ____ memory dumps. Hexadecimal
____ is a description of the functional requirements of a system. Logical design
The PCI system bus consists of ____ wires 98
____ is the specification of the characteristics of the system components necessary to put the logical design into action. Physical design
____ measures how much information can be carried in a given time period over a wired or wireless communication medium, usually measured in bits per second Bandwitch
A(n) ____ is the primary result of systems design, reflecting the decisions made and preparing the way for systems implementation. Design report
The number 10-4 evaluates to ____. 0.00001
____ includes hardware acquisition, programming and software acquisition or development, user preparation, hiring and training of personnel, site and data preparation, installation, testing, start-up, and user acceptance. Systems Implementation
The tendency of a signal to become weaker over distance is known as ____ Attentuation
The preparation of the location of a new system is called ____. Site preparation
A ____ is identified by the number of digits a numbering system has, including the digit 0. Base
____ involves making sure that all files and databases are ready to be used with new computer software and systems. Data conversion
The concept of ____ value exists in all numbering bases. Positional
____ is the final step of systems development, involving the analysis of systems to make sure that they are operating as intended. Systems review
____ media are physical media such as copper wire or fiber-optic cable. Guided
Transistors are simply small electronic switches that can be in either an on or an off state True
Twisting copper wires nearly eliminates the electromagnetic effect known as ____. Inductance
According to positional value, the binary number 10012 represents ____ things. 9
A capacitor consists of three parts: an emitter, a collector, and a base False
Fiber-optic cables use ____ fibers to guide light pulses along a cable Glass
____ values are used to calculate how many of something a number represents. Positional
A truth table with two inputs would have two rows False
A ____ value is calculated by raising the base to the power indicated by the position. Positional
____ transmissions are capable of transmission rates up to 4 Mbps Infared
The letters ____ are used in hexadecimal to represent numeric values above 9. A-F
The Boolean OR operator returns a 1 only when either or both of the inputs are 1 True
A(n) ____ is a set of rules designed to facilitate communication Protocol
To use the Windows calculator to convert between hex, binary, and decimal make sure the calculator is in the ____ view. Scientific
The AND operator has an output of 1 (true) only if both inputs are 0 False
A(n) ____ diagram shows the protocol interactions between two entities Timing
A group of 8 bits considered as one unit is known as a ____. Byte
Each 1 and 0 (on and off ) in a computer is referred to as a ____. Bit
The combinations of gates enables the computer to do all the things that it does True
Larger groups of bits (more than 8) are referred to as ____. Words
Each gate in a circuit reacts in a completely unpredictable way False
The OSI conceptual model for the communication process has ____ discrete layers Seven
The OR gate allows two inputs and one output True
The Transport layer of the OSI model uses data units called ____. Datagrams
The NOR gate is a combination of an OR gate and a(n) NAND gate False
When separating these binary digits into groups of four, you must begin grouping on the ____. Right
In the adder, the bits are added according to the rules of the hexadecimal numbering system False
The ____ layer of the OSI model is responsible for formatting data so that it’s ready for presentation to an application Presentation
When converting from hex to binary, each hexadecimal digit corresponds to a(n) ____-bit binary pattern. 4
Decoder circuits perform functions such as addressing memory and selecting I/O devices True
The flip-flop circuit latches onto a bit and maintains the output state until it’s changed True
Each layer in the OSI model is defined in terms of a(n) ____ and a protocol data unit Header
____ complement is a method of representing negative numbers in a computer system. Twos
To find the ____ of a given bit, flip it to the opposite state. Complement
Boolean expressions cannot be used to accurately predict what a circuit will do False
The computer has a crystal clock called a control clock that times, or synchronizes, each of the steps in the fetch-execute cycle False
A ____ is a small number of computers connected together in close proximity, usually in a building or complex. LAN
In scientific notation, the method of displaying numbers uses a(n) ____ and an exponent. Mantissa
Control wires contain the binary data that is being read from or written to memory and I/0 False
Network configurations are often referred to as network ____ Topologies
The eight bit extended ASCII (American Standard Code for Information Interchange) character set can be used to represent ____ characters. 256
Bus wires are divided into six separate signal groups False
A ____ is the smallest unit that can be displayed on a computer monitor. Pixel
The bus signal group data contains a clock-timing signal for the bus as well as other wires pertaining to timing and the bus protocol False
In a ____ topology, a computers or a network device serves as a central point, or hub, for all message Star
Each pixel is stored in the computer as a(n) ____ pattern containing information about its color and brightness. Binary
ROM isn’t erased when the computer power goes off True
Unicode character representation uses a(n) ____-bit standard. 16
RAM is volatile, meaning that when the power goes off, RAM is cleared True
The most commonly used form of mass storage is the floppy disk drive False
A widely used technology that has become an industry standard for LANs is ____ Ethernet
JPG and GIF are examples of ____ image formats. Compressed
Show full summary Hide full summary

Similar

Characters in "King Lear"
eleanor.gregory
med chem 2
lola_smily
Math's Core 1
mitchcharlie
Business Studies - KEY TERMS
Dani Whitrick
GCSE PE
alexis.hobbs99
AQA GCSE Physics Unit 2 Mindmap
Gabi Germain
Language Analysis
Connie Theobald
Maths: Formulas for Areas & Perimeters of Shapes
Annan S
Family in the park
Eleuterio Caicedo Valencia
Salem does not remember
Salma Moustafa