Farmersusedifferentkindsofsoilconservationmethodstoprot...

問題詳情：

Farmers use different kinds of soil conservation methods to protect their land from damage by farming and the forces of nature. One important form of soil conservation is the use of windbreaks.  

 Windbreaks are barriers formed by trees and other plants with many leaves. Farmers plant them in lines around their fields. Windbreaks stop the wind from destroying or damaging crops. They are very important for growing grains, such as wheat.

There have been studies done on windbreaks in parts of West Africa. It is found that grain harvests can be 20% higher in fields protected by windbreaks compared to fields without such protection.

However, 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 movements will take place close to the ground. These movements will lift soil into the air where it will be blown away. For this reason, a windbreak is best if it has only 60% to 80% of the trees and plants needed to form a solid line.

An easy role to remember is that windbreaks can protect areas up to ten times the height of the tallest trees in the windbreaks. 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 shorter trees and other plants with leaves. Locally grown trees and plants are best for windbreaks.

Windbreaks can protect land and crops from the wind. They can also provide wood products. These include wood for fuel and longer pieces for making fences.

 28. If the wind is stopped completely, what will happen? A. The wind will become stronger.    the wind will be kept out. C. Some trees will be pulled up.        D. Some soil will be blown away.

 29. If the trees in the windbreak are 20 meters high, the area being protected is ___________. A. 2000 meters   B. 200 meters   C. 20 meters   D. 2 meters

30. We can learn from the text that ____________. A. windbreaks are popular in Africa B. only trees can be planted in a windbreak C. windbreaks help to produce better harvests

D. the best way to protect soil is building windbreaks 31. The author develops the passage by the following means EXCEPT ____________.     A. following time order       B. making comparison     C. giving examples              D. listing statistics

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