单项选择题
A.the continuous sounding of any fog signal apparatus
B.giving five or more short and rapid blasts of the whistle
C.firing a gun at intervals of about a minute
D.a barrel with burning oil in it，on deck
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?（）.
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（）.