单项选择题
A.from the still water plane to the crest B.from the still water plane to the trough C.from crest to trough D.between water levels at one-quarter of the wave's length
A.navigable semicircle B.dangerous semicircle C.low pressure area D.eye of the storm
A.position and area of the current B.speed and direction toward which the current flows C.type and characteristic of the current's flow D.None of the above