American school life is different from (和……不同) Chinese. American students usually get to school at around 8:30 in the morning. They put their school bags in their lockers (柜子) and only take one book, one notebook and one pen to each class. In class, American students can sit in their seats (座位) when (当……时候) they answer their teachers questions
   At around 12:00 students have lunch. American students don’t go home for lunch. They have it at school. Most students like pizza(比萨), hot dogs or hamburgers. At 1:00 in the afternoon, they start to have classes. Their classes are usually over(结束) at three o’clock. Then they join in the activities （活动）of clubs or sports.

天文学上光年是（　　）