As you can see, the number of cars on roads ________ rising these days．
A.was keepingB.keepC.keepsD.were keeping在线课程C
the number of+可数名词的复数作主语时，中心词是number，谓语动词用单数；a number of+可数名词的复数作主语时，中心词是of后面的名词，谓语动词用复数。时间状语是these days，因此不可能用过去进行时，故排除A和D项；主语是the number of cars，其中心词是number，单数，故谓语动词用单数。