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Ⅰ.阅读理解
Soil conservation(保护) efforts protect soil from wind and water that can blow or wash it away. Good soil produces good crops for both humans and animals.
One important form of soil conservation is the use of windbreaks. Windbreaks stop the wind from blowing soil away. They also keep the wind from destroying or damaging crops. Windbreaks are barriers(屏障) formed by trees and other plants with many leaves. Farmers plant windbreaks in lines around their fields. They are very important when grains such as wheat are grown. Windbreaks are effective(有效的) when a wall of trees and other plants blocks the wind. The windbreaks should also limit violent motions of the wind to those areas closest to the windbreaks.
Windbreaks seem to work best when they allow a little wind to pass through. If the wall of trees and plants stops wind completely, then violent air motions will take place close to the ground. These motions cause the soil to lift up into the air where it will be blown away. For this reason，a windbreak is the best if it has only 60 to 80 percent of the trees and plants needed to make a solid line. An easy rule to remember is that windbreaks can protect areas up to ten times the height of the tallest trees in the windbreak. There should be at least two lines in each windbreak. One line should be large trees. The second line, right next to it, can be formed from shorter trees and other plants with leaves. Windbreaks not only protect land and crops from the wind. They can also provide(提供) wood products. These include wood for fuel and longer pieces for making fences(栅栏). Locally－grown trees and plants are best for windbreaks.
1．Why should windbreaks allow a little wind to pass through?
