We our breakfast when an old man came to the door.We B our breakfast when an old man came to the door.A have just had B had just had
问题描述：
We our breakfast when an old man came to the door.
We B our breakfast when an old man came to the door.
A have just had
B had just had
答
因为Came to the door已经是过去的事情了，过去的过去当然要用过去完成时
答
因为后面是old man came to the door
老人是在过去进了门，而我们是在老人进门之前吃完了早餐，是过去的过去，所以用过去完成时。
答
因为这里后面有when an old man came to the door,则此时吃早饭是在过去发生的老人进门后的一个动作.对与我们老人进门已经是过去了,而吃早饭可以理解为过去的过去.
于是,A中的have just had无法表达过去的过去,是现在完成时
而B中的had just had 正是为过去的过去而准备的,这个句子应该是过去完成时,所以选B