21. --- The Chinese government is trying to protect the giant panda.
--- I know. By 2020, they _________ about 69,500 square miles of land to the country’s nature reserve system.
A. add B. are adding C. will add D. will have added
22. It was when I argued with Hannah _____ I told her we were not going to be friends any more.
A. until B. that C. so D. when
23. Everyone queued up before the booking office in silence, having nothing to do but _________.
A. to wait B. waiting C. wait D. waited
