第二部分：知识运用（共两节，满分45分）
单项填空（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.
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. My doctor has put me on a low-salt _____ to reduce the risk of heart disease.
A. food B. diet C. dish D. meal
