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 After launching，an inflatable raft should be kept dry inside by（）.
单项选择题 A vessel in distress should send by radio telephone the two tone alarm signal followed immediately by the（）.
单项选择题 A distress signal（）.
