You’d better take another flight to New York , for the AA 505 Flight ______ have been cancelled. But you should call the airport to make sure. 

If people eat natural plant poisons by mistake, they _______ go to hospital without delay.

---Look at the boy running on the playground. Is it David? ---It ______ be him. I saw him go to the teachers’ office just now.

— I am going to the dentist at 4 o'clock and it is 3：30 now.— Well, if you ______ leave earlier today, please ask the director for permission.

 We all know that we ________ our carbon footprint, but I suspect that many people wouldn't be prepared to go as far as Joan Pick, a retired scientist living in south London.