听力原文： 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 atmosphere were like that of the earth, this might be possible. But in fact, it's mostly carbon dioxide. Apart from that, there are other problems to be overcome. For example, the temperature would have to be raised from 60 below zero to 15 above it. Scientists who study Mars have let out the programme that they can follow. To begin with, they will have to find out whether life has ever existed on the 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 are 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 100,000 years.

Scientists are interested in Mars because ______.

A．people might have to migrate there someday

B．people could live there naturally

C．life has ever existed there

D．its atmosphere is quite different