a:why was he late for

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a:why was he late for school yesterday?
b:he overslept . by the time he got to the bus stop,the bus ___already ___.
a.was ;leaving
b.has ; left
c.would ; leave
d.had ;left

参考答案:

d

公共汽车在到车站之前就来了,所以用过去完成时内。