阅读理解   

    Four girls go to school every day by taxi. One day one of the girls says, "There is a test this morning. Let's get to school late. Then we will not have the test." "What can we tell the teacher?" One of the girls says. "He will be angry. We will need a good excuse." The girls think for a moment, then one of them says, "Let's tell him that our taxi has a flat tire(瘪胎)." "That's a good idea," the other girls say. "We will tell him that." They get to school an hour later. The test is over. "Why are you late?" The teacher asks, "Sit down, one of you in each corner(角落) of the classroom." The four girls do this. Then the teacher says,"Write down a piece of paper the answer to this question: Which tire is flat."