— There'ssomeone knocking on the door. Who it be?— It must be Mary. She promised to be her

问题补充：

— Theressomeone knocking on the door. Who it be?

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

A. must

B. need

C. shall

D. Can

答案：

A. must

B. need

C. shall

(答案→)D. Can

解析：句意：--有人在敲门。可能会是谁呢？--一定是Mary。她答应八点钟到这里。A. must一定，表主观肯定推测；B. need需要；C. shall将会是；D. can可能，表示怀疑、猜测、惊异（主要用于否定句或疑问句中）。故选D。

考点: 考查情态动词的用法。