1. 是复数。
2. 因为pupil是单数形式，而their是复数代词，用来指代多个人的所有物或特征，所以their pupil是复数形式。
3. 类似的例子还有：their books、their pens等。
their pupil是复数么
不是，复数是their pupils
The school has over 850 pupils.
这所小学有850多名学生。
The sick man's pupils were dilated.
病人的瞳孔放大了。
Her pupils were dilated.
她的瞳孔扩大了。
How many pupils does the school have?
这所小学有多少学生？
their pupil是复数么
pupil是可数名词，其复数形式是pupils
their pupil是复数么
their pupils 才是复数，pupil是单数。