选 D。traffic 不可数，不连用不定冠词。又如：

There is heavy traffic in the downtown area tonight. 今晚市区中心交通繁忙。

Heavy traffic poses a problem in many old towns. 交通拥挤是许多旧城镇的难题。

I was late because there was heavy traffic. 我迟到是因为交通拥挤。

Heavy traffic is causing delays on the freeway. 拥挤的交通造成高速公路上长时间的延误。