单项选择题
The external flotation bladder of an immersion suit should be inflated（）.
A.only after two hours in the water B.only after four hours in the water C.before entry into the water D.upon entry into the water
单项选择题 The dumping of refuse in a lock is permitted（）.
单项选择题 The disadvantage of using a heaving line to pass a towline is that（）.
单项选择题 The direction in which a vessel is steered is the course. The path actually followed is the （）.
