They must have been here the day before yesterday,------------?请问若用反义疑问句,应该写什么

题目

They must have been here the day before yesterday,------------?请问若用反义疑问句,应该写什么

答案

They must have been here the day before yesterday,must not they?
前面用了肯定就后面用否定形式,反之前否后肯
must是情态动词,置后,表示不是很确定他们前天是否一定已经到过这里