【题目】Bob called to tell his mother that he couldn’t enter the house, for he ________ his key at school.
A.had left
B.would leave
C.was leaving
D.has left

【答案】A
【解析】句意：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。