问题补充：
The air in mountain areas is that in big cities.
A. as fresh as
B. fresher than
C. as dirty as
D. dirtier than
答案：
A. as fresh as
(答案→)B. fresher than
C. as dirty as
D. dirtier than
解析：此题考察形容词的比较级或原级。fresher than... 通常乡村的空气要比城市中的空气新鲜, 所以排除A,C, D选项, 虽然A选项本身结构是正确的。
