单项选择题
The two most effective generating forces of surface ocean currents are（）.
A.temperature and salinity differences in the water B.wind and density differences in the water C.water depth and underwater topography D.rotation of the Earth and continental interference
单项选择题 The tide is（）.
单项选择题 The tide（）.It’s 2 hours before low water，and（）your present position you will be aground at low water.
单项选择题 The term flood current refers to that time when the water（）.
