3. 我們應該盡力讓它一直保持那樣。 

2. 我們有很多法律來限制空氣和水污染。

四、完成句子

1. 學生不允許在課堂上講話。

8. —Look over there!Tommy,along with his parents,   (plant) trees in the garden.

  —Yes,how happy they are!

7. —I saw Jane and her boyfriend in the park at eight yesterday evening.

  —Impossible. She (chat) with me at my home then.

6. Are we let (hand) in our homework tomorrow,Mr Li?

5. —Do you know what (cause) him to miss the football game?

   —Repairing a new machine.

4. Lisa (choose) to act in that film,wasn’t she?

3. We haven’t decided how we (go) to Shanghai next week.

2. —You look unhappy. What's the matter?

  —I (not hear) from Rose for months. I'm worried about her.