第一节 单项填空（共20小题；每小题1分，满分20分）
21. We can never expect bluer sky unless we create less polluted world.
A. a; a B. a; the C. the; a D. the; the
22. Is it to hold the 2010 World Exhibition _______ makes Shanghai the focus of the world?
A. when B. that C. which D. what
