A、Its chemical elements must be studied. B、Its temperature must be lowered. C、Big spaceships must be built. D、Its atmosphere mus

Science fiction writers have often imagined humans going to live on the 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 is mostly carbon dioxide. Apart from that, there are other problems to be overcome. For example, the temperature would have to be raised from 6 degrees below zero to 15 degrees 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 the planet of 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 a hundred years. It will take a bit longer, though, to transform the atmosphere so that human beings could live there. Scientists estimate this will take one hundred thousand years.
Q. 15. Why are scientists interested in Mars?
Q. 16.What is the one of the things that must be done for a man to live on Mars?
Q.17.Why do scientists want to find out whether there is sufficient nitrogen on Mars?
Q. 18.What is the prospect of people living on Mars?