—Who______ it be that is knocking at the door? —It ______ be Father, but I’m not sure.
The child was __________ at the __________ characters in the movie.
Her father is said __________ abroad. When do you know he left?
__________ he took part in the competition, he won a second prize.
I still remember __________ chess in class by my teacher about twenty years ago.
As is known, mobile phones make it possible for people to __________ each other conveniently.