问题描述：
The flight ____ because of the snowstorm,many passengers could do nothing but take the train
a.had been canceled b.have been canceled
c.were canceled d.having been canceled
选D 怎么分析?WHY?
a.had been canceled b.have been canceled
c.were canceled d.having been canceled
选D 怎么分析?WHY?
问题解答：
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