There was a place in ancient times, there were only two kinds of people, knights and rogues. The knights told the truth and the rogues told the truth, but there was no difference in appearance. A scholar met two people, A and B. He asked A: " There must be a knight among you two? "A said," No. ". Can you judge who A and B are? ['A is a knight and B is a rogue.', 'A. B are both knights.', 'A is a rogue, B is a knight.', 'A. B are both rogues.']