单项选择题
An alternating light（）.
A.shows a light with varying lengths of the lighted period
B.shows a light that changes color
C.marks an alternate lesser-used channel
D.is used as a replacement for another light
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All persons or vessels within the lock area，including the lock approach channels，come under the authority of the（）.
A.dockmaster
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D.lock foreman -
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A vessel which is being assisted by an icebreaker would display a flag hoist consisting of the code numeral 4 when the vessel（）.
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B.stops
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A.displaying code letter Y (Yankee)
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D.sounding one prolonged blast on his whistle
