关于英语定语从句的关系词

关于英语定语从句的关系词
Can you take us to the room ________ the important meeting was being held?
空格处是填 where 还是 in which?
我的意思是如果在从句中缺介词的话,前面能直接用where吗?还是一定要用 介词+which
另外 The river ______ we used to swim in has become one ________ no fish live.
每空一字
这儿的第一空 which和where都可以么 第二空呢？