--- Listen! David is talking happily with his mother in the next room. --- It ______ be his mother. She has gone to Yunnan to be a volunteer doctor.
Jack’s grandfather ________for more than twenty years.
They went on working ________the heavy rain.
--- Have you seen the film Titanic? ---No , but I ________.
---- Is Ann knocking at the door? --- No, it ______ be Ann. She has gone to Beijing.
I tried several jackets on ,but _____of them fit me well.