第一节：语法和词汇知识(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。
例：We ________ last night, but we went to the concert instead.
A. must have studied B. might study C. should have studied D. would study
答案是C。
21. It is reported that 1.4 million people took part in ________ post-graduate entrance exam, ________ 13 percent increase from last year.
A. a; the B. the; a C. a; / D. /; a
22. You really don’t need to buy another computer ________ you have already got one.
A. while B. before C. though D. when
