题目内容：

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

优质解答

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