1.C where = in which
the people are really friendly in those places
2.先把它还原成陈述句：
This is the factory ________ you.
the factory是定语从句修饰的先行词,visit the factory不需要介词,所以直接用关系代词that/which
this factory is _________ some.
横线上缺定语从句修饰的先行词,因为先行词一定是名词,所以选the one
3.where = in which
he worked in the factory ten years ago.
4.where = in which
the wolves couldn't be found in the places.
当定语从句修饰一个表示地点的名词时,不缺介词用which,缺介词用where