If he ____ the book before, Jack would be able to tell us something about the story now.
Students ______ remain in their seats until all the exam papers have been collected.
The atmosphere of the earth ______ us from the strong sun light.
---How well did John do in the match? --Wonderful. I think we can’t have a ______ player in our football team.
Many countries are increasing their use of natural gas , wind and other forms of ______.
The fact that many college graduates get lower pay than some workers has ______ a heated discussion.