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。