... cause it and are also its (48) . People cutdown trees for fuels and farmland. Their herds eat up the grass (49) valley floors and hillsides. Theclimate and soil quality, (50) , are effected and ... became dry and sand burried their houses and crops.(58) , governments in nearby areas have had local people palnt trees and banned burningfirewood (59) fuels. Even Beijing is threatened by sandstorms ... C.dunes D.slopes16. When leaves fall from a tree, they make the land richer. When trees are gone, the smaller plants die and the land becomes . A.empty B.roten C.steep D.desert17. The Sahara...