单项选择题
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
单项选择题 All persons or vessels within the lock area，including the lock approach channels，come under the authority of the（）.
单项选择题 A vessel which is being assisted by an icebreaker would display a flag hoist consisting of the code numeral 4 when the vessel（）.
单项选择题 A vessel which is being assisted by an icebreaker should indicate that she is ready to cast off the towline by（）.