岩石被水的作用磨平了。

The rocks are smoothed by the ___action of water.

岩石峡谷被水的作用侵蚀了数百万年。

The rocky canyon has been eroded by the ___action of water over millions of years.

风蚀是气流长期作用的结果。

Wind erosion is a result of the ___action of air currents over time.