If only one is lying, Ben robbed the bank.
If only one is telling the truth, Carl robbed the bank.
Since Dan’s statement was that Ben was lying, exactly one of these two must be telling the truth and the other must be lying. If Ben is telling the truth and Dan is lying, Dan must be the thief, which means that Al is lying and Carl is telling the truth. But now we have two truthtellers and two liars. So this cannot be right.
Therefore, if there is only one liar, it must be Ben. Al can only be telling the truth if Ben was the robber. If there is only one truthteller, it must be Dan. Carl can only be lying if Carl was the robber.
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