--- This must be a footprint of a tiger. --- It ____ be. A tiger’s should be much larger and deeper than this. 

--Who’s singing in the next room? Is it Lucy?---It       be her. She has gone to New York.

---Who is the tallest boy?      ---It ______ be John.. John is of medium height.

 Jack _______ swim when he was five years old. 

She _____ your birthday party, but she had to look after her mother in hospital．