单项选择题
A.the vapor air mixture is too rich to burn
B.an explosion may occur if a source of ignition is present
C.the vapor air mixture is too lean to bum the upper explosive
D.limit has been exceeded
单项选择题 Before attempting to put out a class “C” fire involving an electric-driven centrifugal pump, you should FIRST （）.
单项选择题 While working in the engine room, you hear seven short blasts followed by one long blast on the ships whistle, supplemented by the same signal on the general alarm bellsYou should （）.
单项选择题 All personnel should be familiar with the lifeboats （）