額..我是不是可以這樣理解,你的 ---- 是下劃線的意思.
如果是這樣,那就是 He ran back to my class room which he had left his books in.
這個(gè)是which作定語(yǔ)從句,因?yàn)閘eft books和class room做連接,所以介詞用in.
如果下劃線不是限制只能填一個(gè)單詞的,或者可以不填的情況：
也可是 He ran back to my class room in which he had left his books.
這里的in which是一個(gè)倒裝的用法,一般是從句特別長(zhǎng)的時(shí)候會(huì)用.
或者是 He ran back to my class room where he had left his books.
注意where作連接的話句子末尾就不要加介詞了.
順便說(shuō)一句,是不是你的題目分類(lèi)選錯(cuò)了...