21.(    )Trees        fight air pollution. They are natural air conditioners.

A. should     B. must     C. can

22.(    )—Can you come to my party on Saturday afternoon?

—        .I’ll have to help my parents.

A. Sorry, I can’t   B. Yes, I’d love to  C. Let’s go

23.(    )—Is Jessica giving us a speech this evening?

—No, it          be her. She           to Japan.

A. mustn’t; has gone B. mustn’t; has been  C. can’t; has gone

24.(    ) —        late for the meeting next time. —Sorry, I won’t.

A. Don’t    B. Don’t be    C. Won’t be

25.(    )—Never touch my computer while I’m away. —          .

A. I shouldn’t   B. I mustn’t    C. I won’t

26.(    )—       to give the letter to Mr. Wang. It’s very important.

—No, I won’t.

A. Remember   B. Be sure    C. Don’t forget

27.(    )—May I put my car here? —Sorry, you mustn’t. Look at the sign. It says

“          ”

A.NO PHOTOS  B. NO PARKING  C. DANGEROUS

28.(    ) “Sorry for being late again.”“       here on time next time, or you’ll be punished.”

A. Be     B. Being    C. To be