二氧化碳并释放氧气。根据句意选D。

4. 解析:选D。考查定语从句。situation表示抽象意义上的地点，从句中不缺少主语、宾语或表语，而缺少状语，故用where引导。故选D。

B. 1. Because　 2. for　 3. Since　 4. As

C. 1. much too　 2. too much

7. urgent　 8. frightening 9. protection　 10. recycle

10. -Hurry up. We are supposed to reach the station before 7:00.

-　　　 . The traffic in this city is heavy during the rush hour.

A. That’s definite

B. It’s absolutely hopeless

C. I have no idea

D. It’s scary

9. The heaviest flood of all time struck Chongqing city last year and large amounts of farmland　　　 .

A. has been ruined

B. had ruined

C. have been ruined

D. had been ruined

8. -What do you think are the main environmental problems for us now?

-　　　 , the most urgent problem of all is global warming.

A. After all

B. In a nutshell

C. By the way

D. What’s more

7. Bill told us his terrible experience 　　　in a heavy rain.

A. to catch

B. to be catching

C. to have been caught

D. to have caught

6. When I showed my students the photos taken in the USA, they couldn’t help 　　　the wonderful life there.

A. imagine　　　　 B. to imagine　　　 C. but imagine　 D. but to imagine