21. Unfortunately, when I dropped in, Doctor Li _____ for Beijing to join in the fight against SARS, so we had not enough time for a talk.
A. just left B. has just left
C. is just leaving D. was just leaving
22. —You've made great progress in your English study, haven't you?
—Yes, but much.
A. remains to do B. is remained to do
C. remains to be done D. is remained to be done
23. — You can't finish the book in less than an hour, I suppose?
—.
A. Yes, I'm sure I can B. No, hardly
C. Sorry, I can't D. I don't think I can
24. The twins，_______ so long，held each other and burst into tears．
A．being separated B．having been separated
C．having separated D．had been separated