--Jack, is this your jacket?--No, it _______ be my jacket. My jacket is black but this one is blue.
--- Lily is ill in hospital .---Really ? I _____ know she was ill .
The task is so hard for e . I don’t know _______________.
Good books help us ___________ a whole new world to me .
There is no air _______ water on the moon .
They don’t know what ___________ the problem .