private cars are bringing us convenience they may also cause more traffic accidents and

问题补充：

private cars are bringing us convenience, they may also cause more traffic accidents and pollution.

A. While

B. As

C. If

D. Since

答案：

(答案→)A. While

B. As

C. If

D. Since

解析：while表示对比，意思接近whereas“当……却……”。