单项选择题
Tides are superimposed on nontidal rising and falling water levels，caused by（）．
A.freshets B.floods C.river flows D.seismic events
单项选择题 The set of the current is the（）.
单项选择题 The Benguela Current flows in a（）.
单项选择题 Stand of the tide is that time when（）.
