问题补充：
Is this factory your sister used to work?
A. that
B. the one which
C. where
D. the one where
答案：
A. that
B. the one which
C. where
(答案→)D. the one where
解析：考察定语从句，该句的陈述句为this factory is your sister used to work?
选择D，是定语从句，先行词为the one(即 this factory），在从句中作地点状语。故选择where =in which
