—Who____ it be that is knocking at the door? —It ____ be Father, but I’m not sure.
It’s the first time ______ he has helped us.
Here are two pairs of shoes, ______ you may choose.
He plays well or badly according to his mood (心情), so he’s a very _________ player.
No one knows exactly ______ this important discovery was made.
The weather turned out to be very good, _______ was more than we could expect.