第一节 单项填空（共15小题；每小题1分，满分15分）
认真阅读下面各题，从题中所给的A、B、C、D四个选项中，选出最佳选项，并在答题卡上将该项涂黑。
例: It is generally considered unwise to give a child ________ he or she wants.
A. however B. whatever C. whichever D. whenever
答案是B。
21. Lessons can be learned to face the future, _______ history cannot be changed.
A. though B. as C. since D. unless
22. The book has helped me greatly in my daily communication, especially at work _______ a good impression is a must.
A. which B. when C. as D. where
