my answer, "which one is the bedroom?", was graded incorrect. However, I understood the question as if it was asking to clarify which one was the bedroom (or dormitory) out of other rooms. Hopefully I make sense... :)
Depends on how precise you want to be. For learning purposes, I think you should be as much as you can. "Dormitory is a specific type of bedroom, destined for a bunch of people, as in a boarding school, a barrack or something like that.