最常见的生殖细胞肿瘤为 A．无性细胞肿瘤 B．胚胎性癌 C．卵黄囊瘤 D．未成熟性畸

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 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 in parts of West Africa , for example . These found that grain harvests can be twenty percent 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 motions will take place close to the ground . These motions will lift soil into the air where it will be blown away .
For this reason , a windbreaks is best if it has only sixty to eighty percent of the trees and plants needed to make 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 .
These 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 .
1.What is the best title for the passage?

A.Building a windbreak to protect soil	B.Building a windbreak to protect crops
C.Using a windbreak to improve grains	D.Using a windbreak to improve plants

2.In the fifth paragraph the author tells us windbreaks______.

A.do not work at all	B.do not stop wind fully
C.will not take place	D.will not allow wind to pass through

3.According to the passage , which of the following statement is wrong ?
A.Windbreaks are barriers .
B.Windbreaks can be used to make fences .
C.In general , there are two lines in all in every windbreak .
D.Windbreaks can protect areas .