Question

--- _______ we clean the classroom at once?  

--- No, you _______. You _______ clean it after school

1. A.
   Must; needn’t; may
2. B.
   Must; mustn’t; can
3. C.
   Shall; can’t; must
4. D.
   Need; mustn’t; may

Answer 1

A
用must提问的句子的否定回答，一般用needn’t。所以可以排除其他选项，选a。