—Someone is knocking at the door. Is it Ann?—It be her. She is giving a performance at th

问题补充：

—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

答案：

A. may

B. must

(答案→)C. can’t

D. mustn’t

解析：may 可以，可能，表示推测与其；must必须，一定；can’t不可能，可能不；mustn’t禁止。句意：有人正在敲门。是安吗？联系下文，现在她正在剧院演出。可知前文指的是不可能是她。故选Ｃ。