单选题 On a rainy day, you will see many streams of muddy(多泥的) water running down the slopes(斜坡) of a hill. The water is muddy because it washes away soil from the hill slopes.
Sometimes soil is blown away by strong winds. When the soil is carried away by water or wind, we say that the land is eroded(侵蚀). This is known as soil erosion.
Plants cannot grow on eroded land. There is not enough soil on eroded land to give them the things they need. Plants need water and salts from the soil.
We can do a number of things to stop soil erosion. This is called soil conservation(保护).
One way of soil conservation on flat, open ground is to grow small plants such as grasses. Their roots hold the soil tightly together.
Another way is to plant trees around an open field. Thus soil erosion by strong winds cannot take place. Rows of trees act as a very big wall.
Soil erosion on slopes can be stopped by cutting "steps" called terraces (梯田). Water carrying soil cannot run straight down the slope now. It has to run down the terraces. This slows down the flow(流速) of the water. Most of the soil in the water is left behind on the terraces.
There are some other ways for soil conservation.

单选题 On a rainy day the water running down the slopes of a hill is muddy because ______.

单选题 Plants cannot grow on eroded land because ______.

单选题 The way of soil conservation on flat, open ground is ______.

单选题 Cutting "steps" called terraces on slopes ______.