Radioactivity is...

Types of radioactivty:
A) Alpha (it consists of helium nuclei deprived of electrons which unstable nuclei shoot away)
B) Beta (it consists of electrons or else a special sort of electron but positively charged called positron)
C) Gamma (gamma rays are electromagnetic waves with high energy and extreme power of ionization
Match the barrier which is able to absorb each type of radiation with the radiation alpha, beta, gamma:
Thin barriers absorbs it
Medium barriers stops it
Even very thick barriers hardly stops it