问题补充：
—Where is Jack?I can’t find him anywhere.
—He his homework upstairs.
A. might have done
B. must have done
C. must be doing
D. must do
答案：
A. might have done
B. must have done
(答案→)C. must be doing
D. must do
解析：must be doing是对目前正在发生的事进行推测。
