There are two disadvantages to CO2 pump as a firefighting agent. One of these is the limited quantity available，and the other is（）.
A.the lack of cooling effect on heated materials
B.that it cannot be used in a dead ship situation with no electrical power to the CO2 pump
C.that it breaks down under extreme heat to form poisonous gases
D.there is no effect on a class A fire even in an enclosed space