---- Is Ann knocking at the door?--- No, it ______ be Ann. She has gone to Beijing. 

The boy is old enough to ---------himself.

The problem is too difficult for him. He _______ solve it by himself without any help.

 —Do you think we are going the right way to the Forbidden City?—Sorry, I’m not sure．But it _______ be．

----Did you go to the cinema last night? ----No, We    , but our car broke down on the way.