填入一个适当的词补全对话，每空一词。

A: Hi, Jill. What are you  these days?

B: I'm reading a book about the future.

A: Sounds cool. So what does it write about?

B: It's about what the future will be .

A: Can you tell me something about it?

B: Of course. Cities will be more crowded and polluted. There'll be fewer trees and the environment will be in danger.

A: That sounds bad!  can we do with that?

B: Maybe we'll have to move to other planets.

A: But I want to live on the earth.

B: Me, too. So we must do something to save our earth now.

A: I agree with you. We can use less water and plant  trees.

B: I think everyone should play a  in saving the earth.