复数

例句：

1. 1. The number of students in the class is increasing.
   班上的学生人数在增加。
2. 2. There are five apples on the table.
   桌子上有五个苹果。
3. 3. The teacher asked us to count the books in the library.
   老师让我们数一下图书馆里的书。