They       have got lost, or had an accident.

--____________ you sing an English song?--- Yes, I can.

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

－Look, your sister is playing the piano over there.－She ______ be my sister. She has gone to America.

For those who            go to college, there are still hundreds of high-paying jobs that only require a high school diploma.