答案：A in the house where
一、句子分析：
主句：I found him in the house
定語從句：where he was living.(where = in the house)
二、I found him in the house where he was living 可以化為簡(jiǎn)單句：
1.I found him in the house 我在那個(gè)房子里找到他.
2.He was living in the house.他正住在那個(gè)房子里.
三、用 house 作為先行詞合成定語從句：
1.如果用關(guān)系代詞 which 指代 house,那么就可以變成：I found him in the house (in which) he was living..
2.因?yàn)?in which = where,于是就出現(xiàn)了題目中所出現(xiàn)的情況：I found him in the house (where) he was living