阅读下列短文，从每题所给的四个选项（A、B、C和D）中，选出最佳选项。
A
At first sight the planet Mars（火星）does not appear very welcoming to any kind of life. It has very little oxygen and water, the temperature at night is below-50 degrees and winds of 100 miles （161 kilometres）per hour cause severe dust storms. However, the surface of the planet seems to show that water flowed across it at some time in the past, and it is believed that there is enough ice at the poles to cover the planet with water if it melted. Although there is no life on Mars now, some scientists think that there may have been some form of life a long time ago. At that time, the planet had active volcanoes（火山）; the atmosphere was thicker and warmer; and there was water. In fact, in some ways Mars may have been similar to Earth, where life exists.
Some people believe that Mars could support life in the future if the right conditions were produced. The first step would be to warm the planet using certain gases which trap the sun’s heat in the planet’s atmosphere. With
