单项选择题
A condition where two currents meet at the downstream end of a middle bar can be determined by a（）.
A.small whirlpool B.smooth patch of water C.V-shaped ripple with the point of the V pointing downstream D.V-shaped ripple with the point of the V pointing upstream
单项选择题 A coastal current（）.
单项选择题 A backlash below a lock is defined as（）.
