Question

--- Must I water the flowers right now?
--- No, you           . You may do it after supper

1. A.
   mustn't
2. B.
   needn't
3. C.
   can't

Answer 1

B
must的含义是必须一定，其否定形式mustn’t表示禁止的含义。故must的否定回答一般借助于needn’和don’t have to来构成，句意:我必须立即浇花吗？——不，你不必。你可以晚饭后再浇。结合语境可知选B。
考点：must的否定回答