When Becky ____ after school, she saw her parents in the living-room.
By the time of last year, she ______ the piano for five years.
Tom likes the book ______ she borrowed from Jim.
—Listen! Peter is singing in the classroom! —It _____ be him. He has gone back to his hometown.
Keys _______ opening the lock in the door.
—I have never been to the Great wall. —If you have an opportunity _______ there, you must take it.