Four boys were at the same school. They were good friends, so they often studied and played together. They went to school every day by taxi. One day one of the boys said, "There is a test this morning. Let's go to school late. Then we won't have to take the test." "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 ." "That's a good idea," the other boys said. "We'll tell him that." They arrived at school an hour later. The test was finished. "Why are you late?" the teacher asked, "You miss the test." "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 the answer to this question on a piece of paper: Which tire was flat?" One day the four boys were late for school because  _  .
their taxi had a flat tire they wanted to miss the test the traffic was busy they didn't like the teacher's class