53.We _______ our breakfast when an old man came to the door.　　　　 A)just have had 　　　　 B)have just had 　　　　 C)just had 　　　　 D)had just had

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选 D. 后面已经有用过去时， 吃饭是发生在过去的过去，所以用过去完成时。