What are you doing? Don't you know that you _____ ...

★ Grammar - may/might/could

What are you doing? Don’t you know that you _____ write on the desks?

A. don’t have to

B. mustn’t

C. needn’t

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I _____ come to the concert tonight, but I’ll give you a call later and tell you for sure.

A. may

B. can

C. must

D. could