2. 外语
  3. At the first sight the planet Mars does not appear very welcoming to 【S1】______. any kind of life. It has very little oxyg

At the first sight the planet Mars does not appear very welcoming to 【S1】______. any kind of life. It has very little oxyg

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问题     At the first sight the planet Mars does not appear very welcoming to       【S1】______.
any kind of life. It has very little oxygen and water, the temperature at         
night is below -50 degrees and winds of 100 miles per hour cause severe         
dusty storms.  However, the surface of the planet seems to show that           【S2】______.
water flowed across at some time in the past, and it is believed that there    【S3】______.
is enough ice at the poles to cover the planet with water if il melts.         【S4】______.
Although there is no life on Mars. some scientists think that there may         
have some form of life a long time ago. At that time, the planet had active    【S5】______.
volcanoes; the atmosphere was thinner and warmer; and there was water.         【S6】______.
In fact, in some ways Mars may have been different to earth, where life        【S7】______.
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 used certain gases which trap the sun’s heat in the planet’s            【S8】______.
atmosphere. By warmth, water and carbon dioxide, simple plants could           【S9】______.
begin to grow,  These plants could slowly make Mars habitable. It is　　　　　　　　　
established that the whole process might spend between 100,000 and 200,000     【S10】______.
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 Mars and the problems connected with living there.
【S5】
选项
答案在have之后加been
解析 此处为there be结构，根据时间状语a long time ago，作者在对过去可能曾经有过生命形式的一种推测。
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