—I think the man over there must be Bob.—It ______ be him. He has gone to Paris.

—Look! The man at the gate ______ be our headmaster. He is always standing there every morning.—No, it ______be him. He is holding a meeting in the office now.

It was playing video games that took the boy too much time that he          have spent learning．

The door ________ open, no matter how hard she pushed. 

As students, we ________ study hard to make our country more beautiful and stronger.