Identify from the following list, which systems are a piping arrangements for steam heating systems.
1. Hartford Loop
2. Two-pipe with variable vacuum system
3. One-pipe gravity system
4. Two-pipe return trap system
A) 1, 2, 3, 4
B) 1, 2, 3
C) 2, 3, 4
D) 1, 3, 4
E) 1, 2, 4
Wähle eine der folgenden: