It is 12pm,will you have lunch here?

题目

It is 12pm,will you have lunch here?
sorry,_____ I have many things to do.
A I must not
B I can not
C I don't
D I needn't
为什么不选B呢?

答案

A
must not有不允许的意思,句中因为我有很多事情要做所以不允许也不能我现在吃饭