单项选择题
Scouring is the result of（）.
A.interruption of the normal current flow by the footing B.elevating in mud-slide zones C.penetrating rapidly through a hard crust into weaker soils D.extracting legs from soils with high cohesion
单项选择题 River currents tend to（）.
单项选择题 Priming of the tides occurs（）.
单项选择题 One of the causes of ocean currents is density differences in the water. This is true because （）.
