单项选择题
How do you operate a portable CO2 fire extinguisher?（）.
A.Point the horn down B.Turn cylinder upside-down C.Break the rupture disc D.Pull pin，squeeze grip
单项选择题 High-velocity fog（）.
单项选择题 Halon from extinguishers used on a class B fire，should be directed（）.
单项选择题 Halon fire extinguishers are NOT effective when used on which types of fires?（）
