The length of a wave is the length（）.

A.at right angles to the flow of the current
B.against the flow of the current
C.in the same direction as the flow of the current
D.over slack water

A.weak pressure gradients and light，variable winds
B.the formation of typhoons or hurricanes in certain seasons
C.steady winds in one direction for six months followed by wind reversal for the next six months
D.steady winds generally from the southeast in the Southern Hemisphere

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