有关Must、need等情态动词的一道英语题。详见问题补充：

有关Must、need等情态动词的一道英语题。详见问题补充：
--( Must / Shall / Need ) we clean the blackboard at once?
--No,you ( needn't / mustn't / can't ). You ( may / can / must ) clean it after class.
注意是多选题。我觉得以下几种说法都对：
--Must / Shall we clean the blackboard at once?
--No,you needn't . You may / can clean it after class.
是这样吗？