—Why was he late for school yesterday?— He overslept. By the time he got to the bus stop

问题补充：

—Why was he late for school yesterday?

— He overslept. By the time he got to the bus stop, the bus already .

A. has; left

B. had; left

C. was; leaving

D. have; left

答案：

A. has; left

(答案→)B. had; left

C. was; leaving

D. have; left

解析：句意：昨天他为什么迟到了？——他睡过头了。他到达公交车站时，公交车已经开走了。结合语境可知下文描述的是一直持续到过去某时的动作，故用过去完成时态，选B。