单项选择题
A.vessel towing astern
B.barge pushed ahead
C.vessel towing astern only when the length of her tow exceeds 200 meters
D.this signal does not exist in international waters
单项选择题 You hear the fog signal of another vessel forward of your beam.Risk of collision may exist.You MUST（）.
单项选择题 You are underway in reduced visibility.You hear the fog signal of another vessel about 20° on your starboard bow.Risk of collision may exist.You should（）.
单项选择题 You are underway and approaching a bend in the channel where vessels approaching from the opposite direction cannot be seen．You should sound（）.