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

问题补充：

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

conditions need .

A. when; improving

B. which; to be improved

C. where; improving

D. that; to be improved

答案：

A. when; improving

B. which; to be improved

C. where; improving

(答案→)D. that; to be improved

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