Bob called to tell his mother that he couldn’t enter the house, for he his key at school.

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(答案→)A. had left
B. would leave
C. was leaving
D. has left
解析：该句句意为Bob打电话给他的妈妈，告诉她说他进不了房门，因为他把他的钥匙忘在学校了。根据句意可知，把钥匙忘在学校（leave）的动作发生在打电话（called）之前，called 是过去时，那么忘记钥匙在学校leave就是发生在过去的过去，即过去完成时，故选择A项。
【考点定位】考察时态。