In order to build a road,the old houses ___ used to stand in

In order to build a road,the old houses ___ used to stand in front of our classroom.
,请问是选B吗?但为什么不选 A呢?
In order to build a road,the old houses ___ used to stand in front of our classroom building have already been pulled down.
A:where B:that C:who D:what