阅读理解。 How to Build a Windbreak to Protect Soil Farmers use different kinds of soil conservation methods to protect their land against damage from farming and the forces of nature. One important form of soil conservation is the use of windbreaks. Windbreaks are barriers (栅栏) formed by trees and other plants with many leaves. Farmers plant them in lines around their fields. Windbreaks stop the wind from blowing soil away. They also keep the wind from destroying or damaging crops. They are very important for growing grains, such as wheat. There have been studies done on windbreaks. Studies in parts of West Africa, for example, found that grain harvests can be twenty percent higher in fields protected by windbreaks. This was compared to fields without such protection. Windbreaks can help protect a farmer's land. However, windbreaks seem to work best when they allow a little wind to pass through. If the wall of trees and plants blocks the 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 best if it has only sixty to eighty 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 shorter trees and other plants with leaves. Locally grown trees and plants are best for windbreaks. Windbreaks not only protect land and crops from the wind. They can also provide wood products. These include wood for fuel and longer pieces for making fences. 1. What does the underlined word 'conservation' mean in the passage? [     ] A. Damage. B. Protection. C. Force. D. Harvest. 2. What do farmers use windbreaks for? [     ] A. To keep animals from getting into the fields. B. To keep water in their fields. C. To prevent their soil blowing away and wind destroying the crops. D. To get more trees. 3. According to the passage what is a best windbreak? [     ] A. One with no breaks between trees or plants. B. One with plants which have many leaves. C. One with certain breaks between trees and plants. D. One with as many trees or plants as possible. 4. What's the purpose of the writer in writing the passage? [     ] A. To tell us why farmers build windbreaks around the fields. B. To tell us how to get a higher harvest. C. To let us know there are usually windbreaks around the fields. D. To tell us how important it is to keep soil and crops.