21.　 Gases from cars are ________ (污染) the air of our cities.

20.　 No one goes to school during the ________ (假期)

19.　 The two countries _________ (联合) together to fight against their enemies.

18.　 Doing some part-time jobs can increase the children’s sense of ________ (责任感) for money.

17.　 These animals were returned to _________ (自然界).

16.　 That was a very _______ (冒险) experience.

15.　 Can you tell the _________ (相似之处) and the differences as well.

14.　 Please give my best ______ (祝愿)to your parents.

13.　 Means of ________ (运输) are changing all the time.

12.　 Can you tell me the correct ________ (目的地) of Fight 245?