At the first sight the planet Mars does not appear very welcome to any kind of life．It has very little oxygen and water,the temperature at night is below-50~Cand winds of 100 miles per hour cause severe dust storms．However,the surface of the planet seems to show that water flowed across it 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 melts．Although there is no life on the Mars now,some scientists think that there may have been some form of life long time ago．At that time,the planet had active volcanoes;the atmosphere was thicker and warmer;and there was water．In fact,in some ways the Mars may have been similar to the Earth,where life exists.
Some people believe that the 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 warmth,water and carbon dioxide(二氧化碳),simple plants could begin to grow．These plants could slowly make the Mars more suitable for living．It is estimated that the whole process might take between 100,000 and 200,000 years．In the meantime,people could begin to live on the planet in special closed environments．They would provide a lot of useful information about conditions on the Mars and the problems connected with living there.
Some scientists think there may have been life on the Mars in the past because__.
- A.there is no life there now
- B.there is a large amount of water at the poles
- C.the Mars may be able to support life in the future
- D.conditions may have been similar to those on the Earth
正确答案及解析
正确答案
解析
第一段大意是：人们最初对火星的印象是火星上不可能有生命。因为几乎没有氧气和水，并且夜间温度低于-50°C，同时还有每小时l00英里的风速引起的严重的沙尘暴。然而星球的表面却告诉我们，过去的某个时段水曾经从这里流过。人们相信极地有足够的冰，一旦融化便可覆盖整个星球。虽然现在火星上没有生命，但在过去很久很久以前，生命很有可能存在过。由此可看出，生命所需的条件——水和氧——在火星上和在地球上是相似的。所以选项D正确。而选项B说火星上有水是不对的。火星的极地有冰，只有当冰融化后才能使整个星球有水。
