This game isn't fair, as you could change which one you were "thinking" just to make them get a wrong answer.
Furthermore, this depends on people posting in the thread actively - no one can put another set up to be guessed until the person that posted the current choices comes back and says whether or not the answer was correct.
Therefore, this type of game should not be played in forums. *You may play it in a blog or a clan if you like.