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.
A.falls／in B.has been falling／on C.has fallen／at D.is falling／in
单项选择题 The term flood current refers to that time when the water（）.
单项选择题 The steady current circling the globe at about 60°S is the（）.
单项选择题 The speed of an ocean current is dependent on（）.
