一、单项填空：从A、B 、C 、D四个选项中,选出可以填入空白处的最佳答案。（共15小题；每小题1分，满分15分）
21. I wanted to catch early train, but couldn’t get ride to the station.
A. an; the B. /; the C. an; / D. the; a
22. Yao Ming and Liu Xiang, many of us know, come from Shanghai.
A. what B. that C. how D. as
23. Rescuers are for the safety of two men trapped in the mine.
A. grateful B. concerned C. extreme D. frightened
