问题描述：
They couldn't arrive because they were delayed by?
A.heavy traffic B.heavy traffics
C.crowded trafffic.D.crowded traffics.
选什么?为什么?根据语法规则是什么?
A.heavy traffic B.heavy traffics
C.crowded trafffic.D.crowded traffics.
选什么?为什么?根据语法规则是什么?
问题解答：
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