When he knocked at the door, my mother was busy ____ supper

The volunteers take books to a country school on Children’s Day.

Look at theyoung lady in red. Is it Mrs. King?—No, It ________be her. She is wearing a white dress today.

The computer system _____ suddenly while he wassearching for information on the Internet.

_______ homework did we have to do that we had no timeto take a rest.