单项选择题
Due to the hazards involved with Halon extinguishers on a ship，the size II extinguisher may only be used（）.
A.outside B.on class C fires C.in an emergency D.on class B fires
单项选择题 Dry chemical fire extinguishers are effective on which type(s) of fire?（）
单项选择题 Dry chemical extinguishers extinguish class B fires to the greatest extent by（）.
单项选择题 Convection spreads a fire by（）.
