Man can eventually land on Venus only whenA.the algae have done their workB.the atmosphere becomes coolerC.there is oxygenD.it rains there
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
B.the atmosphere becomes cooler
C.there is oxygen
D.it rains there
参考解析
解析：由最后一段中“before man can set foot on Venus it will be necessary for the oxygen to pro．duce rain”可知，只有金星上有降雨，人类才最终有可能在上面着陆。所以正确答案为D。
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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 lus 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 American scientist, Professor Carl Sagan.Sagan believes that before the earth's resources are compleetely exhausted it will be possible to change the atmophere of Venus and so create a new world almost as large as earth itself. The difficult is that Venus is much hotter than the earth and there is only a tiny amount of water there.Sagan proposes that algae organisms that can live in extremely hot or cold atmospheres and at the same time produce oxygen,should be bred in condition similar to those on Venus.As soon as this has been done, the algae will be placed in small rockets. Spaceship will then fly to Venus and fire the rockets into the atmosphere .In a fairly short time, the alge will break down the carbon dioxide into oxygen and carbon.When the algae have done theri work, the atmosphere will become cooler,but befor man can set foot on Venus it will be neccessary for the oxygen to produce rain. The surface of the planet will still be too hot for man to land on it but the rain will eventually fall and in a few years something like earth will be reproduced on Venus.Inte long run, the most insoluble problem caused by population growth on earth will probably be the lack of ______.a.food b.oilc.spaced.resources
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