问题：

[单选] 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 trough