—Forests are very important aren’t they?—Yes. They can prevent the water washing the eart

问题补充：

—Forests are very important, aren’t they?

—Yes. They can prevent the water washing the earth away.

A．for B．to C．from

答案：

解析：句意:森林是重要的，对吗？-是的，他们可以防止水把土壤冲走。prevent sth. from doing sth. 阻止某人做某事。这是一个固定的句型，介词应该用from。for 为了，因为；to 到。