Convenient private cars are for us they may also cause traffic accidents and pollution.A.

问题补充：

Convenient private cars are for us, they may also cause traffic accidents and pollution.

A. while

B. as

C. if

D. since

答案：

A. while

(答案→)B. as

C. if

D. since

解析：考查让步状语从句。形容词/名词/动词+as +主语+谓语，意为“尽管......”。句意“尽管私家车对我们来说很方便，但它们也可能引起交通事故或污染。”本题是让步状语从句。While也用“尽管”的意思，但while引导一个完整的句子。故选B。