Listen! Is Mary singing in the next room?—No. It _______ be Mary.She has gone to Shanghai for an important meeting.
We usually work _____ the day and sleep _____ night.[ ]
Would you like _________a CD _______your birthday?
It will be hard for us to get up in the morning if we________ to bed too late.
When I turned to Tony for help, I found that the more he tried to explain, ___________.