—There's someone knocking on the door．Who it be?
—It must be Mary．She promised to be here at 8：00．
A. must
B. can
C. shall
D. need
网友回答
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。