________ is knocking at the door．

1. A.
   One
2. B.
   Anyone
3. C.
   Someone
4. D.
   Everyone

Someone is knocking at the door. It     my sister. It's time for her to be back.

— Someone is knocking at the door. Is it Ann?

— It _____ be her. She is giving a performance at the theatre now.

A. may              B. must            C. can’t            D. mustn’t

Someone is knocking at the door. It     my sister. It's time for her to be back.

A．can’t be         B．may not be        C．must be           D．mustn’t be