the students have no classes on saturday afternoon的反义疑问句

don't they?为什么是don't they？反义疑问句前肯后否，have构成问句用助动词do，用代词they指代the students不是有NO么，怎么会是肯定句？对不起，没有看到no，正确答案应该是The students have no classes on saturday afternoon，do they？答案是don't they= =原答案肯定错了。正确答案应该是The students have no classes on saturday afternoon，do they？