may表示请求和允许,意为可以.
may的否定回答可用can't,mustn't,may not表示“不可以”、“禁止”、“阻止”.can't,mustn't,may not都可以无条件替换么不可以。从虚拟语气上来说 may最弱 must最强。may not 语气较弱，”可以不”，You may not finish your homework;can't ”不能”; You can't be too serious;问句回答时：Can I go out to play? No, you can't. 或做may问句的回答：May I sit here? No, you can't.;mustn't 语气强，”禁止”:You mustn't step on the grass.选项里这三个都有的话，是不是要分析语境就行了是的。具体情况具体分析。