They happened to__________ for Tianjin

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

参考解析：

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。