A、It influences the surface temperature of Mars. B、It protects living beings from harmful rays. C、It keeps a planet from overhea

Science fiction writers have often imagined human beings going to live on Mars. But these days scientists are taking the idea seriously. It has a great deal to recommend it since it might solve the problem of overcrowding on the earth, but obviously it would not be worth making the effort unless people could live there naturally. If the were like that of Earth, this might be possible. But m fact, it s mostly carbon dioxide(CO2). Apart from that, there are other problems to be overcome. For example, the temperature would have to be raised from 60~C below zero to 15~C above it. Scientists who study Mars have laid down the program that they can follow. To begin with, they will have to find out whether life has ever existed on planet Mars in the past. Secondly, they will have to make a reliable map of its surface, and finally they will have to make a list of the gases. Above all, they will have to discover how much nitrogen it possesses, since nitrogen is four-fifths of the air we breathe. They’re surprisingly optimistic about raising the temperature on Mars and believe it could be done in 200 years. It would take a bit longer, though, to transform the atmosphere so that human beings could live there. Scientists estimate that this will take 100,000years.