20.　 A well-written composition__________ a good choice of words and clear organization among other things.

A. calls on　　　 　 B. calls for　　　 C. calls up　　 　　 D. calls off

19.　 Distinguished guests and friends, welcome to our school! _____the ceremony of the 50th anniversary this morning are our alumni (校友) from home and abroad.

A. Attend　　　　 B. To attend　　　 C. Attending　　 　　　 D. Having attended

18.　 Many people believe we are heading for environmental disaster _____ we basically change the way we live.

A. but　　 　　　　　　　 B. although　 　　　 C. unless 　　　 D. once

17. I try to understand __________ that prevents so many Chinese from being as happy as one might expect.

A. why is it　 　　　　　　　 B. what is it　　 　 C. why it is　 　　　　　　　 D. what it is

16. ---Has Li Lei finished his work today?

--- I have no idea, but he ______ it when I saw him this morning.

A. had done 　　　 B. would do 　　　　 C. was doing 　　　 D. did