这种情况,介词+which=特殊疑问词作引导词.
这里要具体问题具体对待.
for example:
①I went to the town where I was born.
=I went to the town in which I was born.(I was born in the town)
②We were happy when we got together.
=We were happy at which we got together.(we got together at that time)
介词的选择,需要在从句中判断.