—There's someone knocking on the doorWhoit be?—It must be Mary．She promised to be here at

问题补充：

—Theres someone knocking on the door．Whoit be?

—It must be Mary．She promised to be here 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：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B

考点: 考查情态动词的用法。