29.　 n_________ the world of living things in the outdoors.

28.　 e__________ to give something that you have and get something different; trade

27.　 b__________ to get into a ship, or a plane, or any public vehicle.

26.　 s__________ a feeling of great sadness.

25.　 r________ to take, fill the place of

24.　 r______ to say, do, or experience again

23.　 s_______ to go on living, especially after coming close to death.

22.　 P_____ to make the sound of ( a letter , a word,etc.)

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

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