The problem of air pollution is becoming more serious the increasing number of cars.A. as

问题补充：

The problem of air pollution is becoming more serious the increasing number of cars.

A. as for

B. because

C. as a result

D. as a result of

答案：

A. as for

B. because

C. as a result

(答案→)D. as a result of

解析：本题的含义为由于车子数量急剧增加的结果，空气污染的问题变得更加严重，as a result of 表示由于……的结果的含义，故本题选D。