Large volumes of carbon dioxide are safe and effective for fighting fires in enclosed spaces， such as in a pumproom， provided that the（）.

A.persons in the space wear gas masks

B.persons in the space wear damp cloths over their mouths and nostrils

C.ventilation system is secured and all persons leave the space

D.ventilation system is kept operating

正确答案：ventilation system is secured and all persons leave the space