If the population of the earth goes on increasing at its present rate, there will eventually not be enough resources left to sustain life on the planet. By the middle of the 21st century, if present trends continue, we will have used up all the oil that drives our cars, for example. Even if scientists develop new ways of feeding the human race, the crowded conditions on earth will make it necessary for us to look for open space somewhere else. But none of the other planets in our solar system are capable of supporting life at present. One possible solution to the problem, however, has recently been suggested by an American scientist, Professor Carl Sagan.
　　Sagan believes that before the earth's resources are completely exhausted it will be possible to change the atmosphere of Venus (金星) and so create a new world almost as large as earth itself. The difficulty is that Venus is much hotter than the earth and there is only a tiny amount of water there.
　　Sagan proposes that algae (藻类) organisms, which can live in extremely hot or cold atmospheres and at the same time produce oxygen, should be bred in conditions similar to those on Venus. As soon as this has been done, the algae will be placed in small rockets. Spaceships will then fly to Venus and fire the rockets into the atmosphere. In a fairly short time, the algae will break down the carbon dioxide into oxygen and carbon.
　　When the algae have done their work, the atmosphere will become cooler, but before man can set foot on Venus it will be necessary for the oxygen to produce rain. The surface of the planet will still be too hot for men to land on it but the rain will eventually fall and in a few years something like earth will be reproduced on Venus.

　　21. In the long run, the most difficult problem caused by population growth on earth will probably be the lack of______.
　　A. food B. oil
　　C. space D. resources

　　22. Carl Sagan believes that Venus might be colonized from earth because______.
　　A. it might be possible to change its atmosphere
　　B. its atmosphere is the same as the earth's
　　C. there is a good supply of water on Venus
　　D. the days on Venus are long enough

　　23. On Venus there is a lot of______.
　　A. water B. carbon dioxide
　　C. carbon monoxide D. oxygen

　　24. Algae are plants that can______.
　　A. live in very hot temperatures B. live in very cold temperatures
　　C. manufacture oxygen D. all of the above

　　25. Man can eventually land on Venus only when______.
　　A. the algae have done their work B. the atmosphere becomes cooler
　　C. there is oxygen D. it rains there

　　答案：21. C 22. A 23. B 24. D 25. D