21. If you can make          the teachers teach yours, you’ll make rapid progress.

A. that           B. what     C which      D. who

22. ____ only 20 minutes to go before the train left, I felt uneasy in the taxi to the railway station.
A. For             B. With                 C. As                    D. Because

23. AIDS is said _________ the biggest health challenge to both men and women in that area next few years.
A£®that it is        B£®to be           C£®that it has been    D£®to have been