单项选择题
A tide is called diurnal when（）.
A.only one high and one low water occur during a lunar day B.the high tide is higher and the low tide is lower than usual C.the high tide and low tide are exactly six hours apart D.two high tides occur during a lunar day
单项选择题 A strong back refers to a（）.
单项选择题 A stretch where the channel changes from one side of theriver to the other is called a（）.
单项选择题 A stream of water immediately surrounding a moving vessel’s hull，flowing in the same direction as the vessel is known as（）.
