fifteen-year-olds和a fifteen-year-old boy
问题描述：
fifteen-year-olds和a fifteen-year-old boy
前者是指所有十五岁的青少年
后者是指一个十五岁的男孩
后者如果把横线去掉是不是要变成a fifteen years old boy?
那前者去掉横线呢?
答
fifteen-year-olds是指所有十五岁的青少年,a fifteen-year-old boy指一个十五岁的男孩.后者去掉横线是要变成a fifteen years old boy,前者去掉后是fifteen years old children