1. This is the room in which he was.2. This?is the room which/that he was in.3. This?is the room where he was.4. This?is the cage?in which a dog was.5.?This?is the cage which/that a dog was in.6.?This?is the cage where a dog was.以上句子都正确。但是应该注意:在 that 和 where 前面不能用介词。