Science fiction writers have often imagined humans going to live on the Mars. But these days, (1)____________________________.
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. (2)_________________________, this might be possible.
But in fact it is mostly carbon dioxide. Apart from that, there are other problems to be overcome. For example, (3)___________________________to 15 degrees above it.
