第一节: 单项填空（共15小题；每小题1分，满分15分）从A、B、C、D四个选项中，选出可以填入空白处的最佳选项。
21. In recent years global warming has becoming concern for people all over the world.
A. the; the B. x; x C. x; a D. the; x
22. Though he told me a second time about it, _____I didn’t understand.
A. but B. still C. however D. but yet
23. They must have enjoyed themselves there, otherwise they so long.
A. can’t have stayed B. wouldn’t have stayed
C. can’t stay D. wouldn’t stay
