1. We __________out by that time that he_________ a thief for a longtime.

1. We __________out by that time that he_________ a thief for a longtime.

这个主要因为语序的问题：
We _____ found____out that he___had been ______ a thief for a longtime by that time .
　　A. had found, had been B. had found, was
　　C. had been D. found, was
译文：我们发现他到那时已经做了一段时间的小偷了。
（到那时是修饰从句的，而不是到那时发现这个动作）
参考：by that time 到那时;节止那个时候 一般与完成时连用.
We had learned 5000 words by that time when we graduated.

答案：C
解答：本句的中文意思是：到那时我们才发现原来他一直是个贼。首先B、D 就不用解释了。至于by that time 所接内容的确是要用完成时，但是本句by that time是属于后面一部分的。因此在后面一空使用过去完成时，再者A答案本身有问题，发现与“他已经做了很久的贼”这两个动作发生时间不一样，怎么能用相同时态呢？