20. -Waiter!

-________?

-I can’t eat this. It’s too salty.

　 　A. Yes, sir　 　　B. What　　　 C. All right　　 D. Pardon

19.It was ______ back home after the experiment.

　　　　 A. not until midnight did he go　　　　　　　　　　　　　　 B. until midnight when he didn’t go

　　　　 C. until midnight that he didn’t go　　　　　 D. not until midnight that he went

18. ---Mary, you clean the blackboard after class, _________?

　 ---Well, but it is Tim who’s on duty today.

　　　　 A. don’t you　　　　　　　 B. shall we　　　　 　 C. won’t you　　　　 D. do you

17．I showed an old friend of mine around the city, 　　　　　surprise, the changes had been so great.

A．whose　　 　　　　　　 B．which 　　　　　　　 C．to whose　　 D．to which