I don’t know who invented _____digital camera, and it’s really _____ most useful invention.

A．the; a

B．the; the

C．a; a

D．不填；不填

—Look! The lights in the teachers' office are still on. Is Mr. Li working?

—No. It _______ be Mr. Li. I saw him leave just now. [ ]

A. may not

B. mustn't

C. can't

D. needn't