They happened to__________ for Tianjin

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问题:

[单选] They happened to__________ for Tianjin when we got there.

A . leave

B . have left

C . leaving

D . had left

参考答案:

B

happen to后接动词原形,所以先排除C、D,而不定式表示的动作发生于过去,故选B。