isn't it
若must表示推测,反意疑问句不能用must,而应根据must后的动词结构采用相应的动词形式：
He must be tired,isn’t he?他一定累了,是吗?
He must have read it,hasn’t [didn’t] he?他一定读过它,是吗?
He must have left yesterday,didn’t he?他昨天一定走了,是吗?