Tom was nine years old and he went to a school near his house. He went there on foot and came back home usually at 4 o'clock. But last Monday he was very late. His mother asked, "Why are you so late today, Tom?" "Because my teacher asked me to stay behind  " Tom answered. "Why did the teacher make you stay behind?" the mother asked again. "Because no one could answer the teacher's question except  me," Tom said. "What was the question?" his mother asked. "The question was 'Who broke   the window of the classroom?'" Tom said. Tom went to school   _  .
on foot by bus by bike by car