单项选择题
After putting on a self-contained breathing apparatus，you open the air supply and hear a continuous ringing of a bell. What does this mean? （）
A.The unit is working properly B.The face mask is not sealed properly C.The air bottle needs to be refilled D.The air supply hose has a leak
单项选择题 According to the Chemical Data Guide，epichlorohydrin is a(n)（）.
单项选择题 A space should be ventilated when（）.
单项选择题 （）is the most liable to spontaneous combustion.
