[Marsh.]

Originally Posted by Benjamin

You would ask one door what the other would say.

So if you asked the lying door (door a) what the truth door (door B) would say the lying door would say the truth door would say A (because he is lying). But if you asked the Truth door what the lying door would say the truth door would say the lying door would tell you to take door A. (he is telling you the truth)

So you would take door B.