I'd like to ask about the variation of the question style.
Would you take a look at the following sentences?
1) Who was this CD produced by?
2) By whom was this CD produced?
3) Whom was this CD produced by?
I think 1) and 2) is correct. 2) is probably formal. However what I'd like to know is whether 3) is still grammatically acceptable or not. When you make a question sentence, do you sometimes use "whom"? Or is "whom" obsolete in the question?