南极上空臭氧空洞的形成是因为( )。

A．太阳光太强

B．南极太冷

C．人类活动中排放大量氟化合物的缘故

D．空气中二氧化碳太多，形成温室效应

All portable fine extinguishers must be capable of being（）.

A.carried by hand to a fire

B.carried or rolled to a fire

C.recharged in the field

D.used on class B fires