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