Doctors say that exercise is important for health, but it _______ be regular exercise. 

Smoking pollutes air. You ______ smoke here.

-- A medical team was sent to Japan by air five hours ago. ---So they_____ have arrived by now. The flight usually takes four hours or so.

It has been announced that candidates ______ remain in their seats until all the papers have been collected. 

 If only I _______ your praise then! For, you see, I lost heart and gave up.