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

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

答案及解析： A

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。