—Who______ it be that is knocking at the door?—It ______ be Father, but I’m not sure.A. can; must B. can; may . must; can D. may; must
It was_______ back home after the experiment.
She_______ an old friend of hers yesterday while she was shopping at the department store.
Mrs Mavis said the aliens took her away so that they could do research _______ her.
Mr Smith will not be able to attend his sons wedding ceremony, because he_______ an important speech.
---The police have been______ the area for quite a long time. ---I’ve also noticed that. What have they been______?