We can never expect bluer sky unless we create less polluted world.A. a；aB. a；theC. the；

We can never expect bluer sky unless we create less polluted world.
A. a；a
B. a；the
C. the；a
D. the；the

(答案→)A. a；aB. a；theC. the；aD. the；the解析：句中sky与world都表示泛指，a表示“一”这个数量，相当于“one”。答案　A

我明天再问问老师，叫他解释下这个问题