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
单项选择题 Tropic tides are caused by the（）.
单项选择题 To make sure of getting the full advantage of a favorable current，you should reach an entrance or strait at what time in relation to the predicted time of the favorable current?（）
单项选择题 Tide is（）in direction ENE.
