单选题 Four boys went to school every day by taxi. One day one of the boys said, "There"s an exam this morning. Let"s go to school late. Then we won"t have to take the exam." "What can we tell the teacher?" one of the boys said, "He"ll be angry. We"ll need a good excuse," The boys thought for several moments, then one of them said, "Let"s tell him that our taxi had a flat tire (轮胎气)." "That"s a good idea," the other boys said, "We"ll tell him that."
They arrived at school an hour late. The exam was finished. "Why are you late?" the teacher asked, "You missed the exam." "Our taxi had a flat tire," one of the boys said. The teacher thought for a moment, then he said, "Sit down, one of you in each corner of the room." The four boys did this. Then the teacher said, "Write on a piece of paper the answer to this question: Which tire was flat?"