单项选择题
A whistle signal of one prolonged，one short，one prolonged and one short blast，is sounded by a vessel（）.
A.at anchor B.towing a submerged object C.being overtaken in a narrow channel D.in distress
单项选择题 A vessel using a traffic separation scheme shall（）.
单项选择题 A vessel sounds one short blast. This signal indicates the vessel（）.
单项选择题 A vessel not under command sounds the same fog signal as a vessel（）.
