Which statement about entry into a water ballast tank that has been sealed for a long time is TRUE?

A
A "buddy system" should be used where someone enters the tank with you.
B
Sea water acts on the ship's metal and generates chlorine gas which may accumulate in poisonous quantities.
C
You should always wear a gas mask.
D
After ventilation and testing, and the tank is found safe for entry, someone should stand by at the tank entrance while you are inside.
E