—Listen! There is a knock on the doorWho it be?—It must be Jack．Just now I sent for him．

问题补充：

—Listen! There is a knock on the door．Who it be?

—It must be Jack．Just now I sent for him．

A. shall

B. can

C. must

D. might

答案：

A. shall

(答案→)B. can

C. must

D. might

解析：本题考查情态动词。句中can表示推测，“可能”之意。句意：听！有人敲门，可能是谁呢？——一定是杰克，刚才我去请他的。