补全对话。
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.