C

    In the United States, headmasters and teachers discipline (惩罚) students in several ways. The teacher often writes to or calls the students' parents. Sometimes students have to stay at school for one hour. If a student behaves very badly, the headmaster can stop the student having classes. The student can't come to school for one, two or three days. Mr. Lazarus, the headmaster of a middle school in Ohio, did not like to do so. When he didn't let the students come to school, they were happy. “A three-day holiday!” they thought.

    One day, a boy was in Mr. Lazarus' office. The boy was not behaving well in class. Mr. Lazarus telephoned the boy's parents. “If you come to school with your son, I won't stop him having classes,＂he said. The boy's father came to school and went with his son to every class. Other students looked at the boy and his father. The boy was embarrassed. After that he behaved better. And, of course, other students behaved better, too.

    Now headmasters all over the USA are trying Mr. Lazarus's idea. They too, think that students behave better when parents come to school.

足球场上，同学们在踢球时肌肉收缩所需要的能量来自于人体细胞的（　　）

他拉着我，坐在一根伐倒的树干上，把这朵花擎在我的眼前，说：“仔细看看，仔细看看，时间已经不多了!”

在这里，“时间已经不多了”是指（    ）