Soil conservation efforts protect soil from wind and water that can blow or wash it away. Good soil produces food crops for both people and animals.
One important form of soil conservation is the use of windbreaks (防风林). 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 blowing soil away. They also keep the wind from destroying or damaging crops. They are very important for growing grains, such as wheat.
For example, in parts of West Africa, studies have shown that grain harvests can be twenty percent higher on fields protected by windbreaks compared to those without such protection.
Windbreaks are effective 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 windbreak.
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 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.
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. Locally-grown trees and plants are best for windbreaks.      
68. What does this text primarily concern?
A. A way to protect soil.          B. Some advice on growing windbreaks.
C. The effect of windbreaks.       D. A way to protect crops.
69. Which of the following is NOT true about windbreaks?
A. If windbreaks stop the wind completely，the soil will be blown away above the windbreaks.
B. Leaving sixty to eighty percent of the trees and plants needed to make a solid line is a best windbreak.
C. A better windbreak should be two lines or more with the same height.
D. Windbreaks work best during an area up to ten times the height of the tallest trees in them.
70. The underlined word "barriers" (Para. 2) refers to         .
A. something that is built with trees together with other plants
B. a structure built to forbid passage of wind with trees or other plants.
C. something used to help things control their movement.
D. narrow lines where trees and other plants with many leaves are planted.
71. Which of the following is most likely to show the right area that a windbreak can protect?