— Who’s singing in the next room? — It ______ be Mary. She ______ New York. 

—It's the office！So you ________ know eating is not allowed here.—Oh，sorry.

    --- Whose backpack is this?--- It        be Mary’s. She is looking for hers.

You       be hungry after the long walk. Help yourself to some cakes.

— Simon, when are we going to have a class meeting this week?— Sorry. Ask our monitor, please. He         know.