There is a new problem involved in the popularity of private cars road conditions require .
A. that ... improving
B. which...to be improved
C. where ...improving
D. when...improving
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(答案→)A. that ... improving
B. which...to be improved
C. where ...improving
D. when...improving
解析：考查同位语从句和固定搭配。involved in the popularity of private cars是problem的后置定语， road conditions require 是problem的同位语从句，讲的是problem的具体内容，且同位语从句意完整，用that引导；某事需要被做，可以用require to do或require to be done 结构。所以选A。