I found him _ he was livingA in the house whereB whereC in the houseD both A and B要理由,

答案：A in the house where
一、句子分析：
主句：I found him in the house
定语从句：where he was living.(where = in the house)
二、I found him in the house where he was living 可以化为简单句：
1.I found him in the house 我在那个房子里找到他.
2.He was living in the house.他正住在那个房子里.
三、用 house 作为先行词合成定语从句：
1.如果用关系代词 which 指代 house,那么就可以变成：I found him in the house (in which) he was living..
2.因为 in which = where,于是就出现了题目中所出现的情况：I found him in the house (where) he was living