单项选择题
Gas masks（）.
A.should be worn while fighting a fire B.can be used in atmospheres deficient in oxygen C.filter contaminants from air that is to be breathed D.may be substituted for a self-contained breathing apparatus
单项选择题 Fusible-link fire dampers are operated by（）.
单项选择题 Foam-type portable fire extinguishers are most useful in combating fires involving（）.
单项选择题 Foam is a very effective smothering agent and（）.
