There is a new problem involved in the popularity of private cars road conditions need to

问题补充：

There is a new problem involved in the popularity of private cars road conditions need to be improved.

A. that

B. which

C. where

D. when

答案：

(答案→)A. that

B. which

C. where

D. when

解析：该句为分隔性同位语从句。that引导同位语从句，作problem的同位语，解释problem的具体内容。