—Why couldn’t they catch the 6:30 train?—Because they were delay

—Why couldn’t they catch the 6:30 train?—Because they were delayed by ____.
A、some heavy traffics
B、heavy traffic
C、a heavy traffic
D、any heavy traffic
【正确答案】：B
【题目解析】：heavy traffic拥挤的交通，traffic是不可数名词，无需加a。