补全对话。（每空一词，共5分）
A: What bad weather! Another sandstorm begins.
B: So it is. But why are there so many sandstorms recently?
A: Because there is so much sand around us.
B: Why is there so much sand?
A: Because a lot of land has been ____ into sand.
B: And why?
A: Because people have _____ down a lot of trees.
B: That may be the root(根源). We should ____ people from cutting down trees. Although we have built “ the Great Green Wall”, we still need to work hard to _____ the environment.
A: I agree ___ you. Let’s go to the hill and plant trees on Sunday.
B:Good idea.