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 for millions of years by the ___action of water.

波浪的持续作用塑造了海岸线。

The constant ___action of waves shapes the coastline.