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One of the causes of ocean currents is density differences in the water. This is true because （）.
A:in an area of high density the water's surface is lower than in an area of low density
B:surface water flows from an area of high density to one of low density
C:the lesser the density gradient the freer the water is to move
D:it is the density differences that cause the currents to stay in the troughs

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出自：海船船员证

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