21. Though our city has a good public transport system, _____private car is becoming _____popular means of transport.

A. a; /                      B. the; the               C. the; a               D. /; the

22. ----Can I smoke here?

-----No, in no circumstances _______in the library.

A. smoking permits                                  B. smoking is permitted

C. does smoking permit                            D. is smoking permitted

23. ----Will you drop in at my house this afternoon if _______?

---- Of course, I will.

A. it is convenient to you                   B. you are convenient

C. it will be convenient to you            D. you will be convenient