— Theres someone outside. Who it be ?— It must be Mary. She promised to come over at 8:

问题补充：

— Theres someone outside. Who it be ?

— It must be Mary. She promised to come over at 8: 00.

A. must

B. can

C. shall

D. need

答案：

A. must

(答案→)B. can

C. shall

D. need

解析：情态动词can 表示推测，用于疑问句和否定句。