单项选择题

Water may boil up around the stern of a vessel in a channel due to（）.

A.slack water when upbound
B.shallow water
C.a cross current
D.a head current

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单项选择题

A.Moon being at its maximum declination
B.Moon crossing the equator
C.Sun and Moon both being near 0°declination
D.Moon being at perigee

单项选择题

A.One hour after the predicted time
B.At the predicted time
C.30 minutes before flood，one hour after an ebb
D.30 minutes before the predicted time

单项选择题

A.getting
B.setting
C.being
D.having