Peter found a place in the cellar ____he used as his first laboratory 填where which it
问题描述：
Peter found a place in the cellar ____he used as his first laboratory 填where which it
答
因为从句中 use 所用的句型是 use...as...,即“将……用作……”,
这里是指“将地下室用作第一个实验室”.
由于从句中的 use 没有宾语,故应选用关系词 that 或 which,
当然也可省略不填（因为用作宾语的关系代词可以省略）.