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（）.
A.10 minutes B.1 minute C.1 hour D.3 minutes
单项选择题 What is the fundamental purpose for imposing radio silence? （）
单项选择题 What is the calling and distress frequency on a single side band (SSB) marine radiotelephone?（）
单项选择题 What is NOT a distress signal?（）
