单项选择题
A.After determining that the frequency or channel appears to be no longer in use
B.After determining that geographic distance from the distress situation will prohibit any other signal from interfering with emergency communications
C.Routine communications can resume after the Rescue Coordination Center transmits a message on the frequency or channel being used for emergency communications stating that such traffic has concluded
D.If，in the Master's opinion，communications on that frequency will interfere with emergency communications
单项选择题 When can routine communications be resumed on a frequency or channel on which radio silence has been imposed? （）
单项选择题 When a vessel signals her distress by means of a gun or other explosive signal，the firing should be at intervals of approximately（）.
单项选择题 What is the fundamental purpose for imposing radio silence? （）