My teacher is very nice．I'll never forget the        he has done for me.

 As many successful businessmen believe, courage is as ______ to pioneering work as the intelligence, even more so．

Carbon dioxide, which makes a ______ between us and the sun, prevents heat from getting out of the atmosphere easily, so the earth is becoming warmer.

The lack of money forced them to________their research.

The mayor will __________ the prizes among the winners.