The sentence finishes with a string of "wh-" noun clauses. These clauses are certainly not questions, so the last 1 should also not be a question. "Which a single would be the best" is obviously a question structure, so it is smart that " To the working day the Vault https://share.evernote.com/note/a8fd1cb2-5733-5366-f5df-7a36578ae666