---Where is Mr. Wu, do you know?---Well, it’s hard to say. But I saw him ___________ a football game just now.
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.