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 (1)_____________________. And finally, they will have to make a list of the gases. Above all, (2)______________________________.
Since nitrogen is four fifths of the air we breathe, they are surprising optimistic about raising the temperature on Mars and believe it could be down in hundred years. It will take a bit longer, though, to transform the atmosphere so that human beings could live there. (3)__________________________.
发表于 2013-9-24 09:55
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发表于 2014-5-10 10:17
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