class mean sample tree method

例句：

1. 1. The teacher is givinga class on the history of the country.
   老师正在给学生们上关于这个国家历史的课。
2. 2. She is a member of the seniorclass.
   她是高年级的学生。
3. 3. Theclass was divided into twogroups for discussion.
   这节课被分成两个小组进行讨论。