—Listen! There is a knock on the door. Who _____ it be?—It must be Jack. Just now I sent for him.

(2013·高考四川卷)—Why are your eyes so red? You________have slept well last night.—Yeah，I stayed up late writing a report.

If you ________ smoke in the room, please keep the windows open.

 —Who’s that boy reading in the study ? Is it David?—It __________ be David. I saw him go home just now.

 Linda _____ come to meet me at the station, but I’m not sure.