What does Mr. Liu usually ___________when he is at school?

Is the man standing outside the teacher’s office Mr. Zhou? –No, it ______ be him. He has gone to Hong Kong for an academic conference.

. The reason why I have to go is _____ if I don’t.

While the alien ________ the museum, the boy called the TV station.

If you are careful enough, you will _________ make mistakes in your exams.