2. 外语
  3. The main reason we explore Mars is to determine if life ever existed on the planet. We have tried for centuries, starting with 【

The main reason we explore Mars is to determine if life ever existed on the planet. We have tried for centuries, starting with 【

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问题     The main reason we explore Mars is to determine if life ever existed on the planet. We have tried for centuries, starting with 【B1】______and lately with satellites, landers and rovers. We have 【B2】______ for it directly and indirectly. No luck so far. Some think life has never been there. Others think it was there in the past but is now gone. Many think why bother, but that’s a whole different story. I think we are 【B3】______in the wrong places. Here is why.
    The surface of Mars is 【B4】______cold and dry. Cosmic and solar 【B5】______reach the surface by the thin atmosphere, and the soil contains strong oxidants (氧化剂)that destroy organic 【B6】______. Fat chance for life. But it wasn’t always so.
    Early in its history Mars was warmer and 【B7】______enough for the development of rivers, lakes and perhaps oceans. The atmosphere was thicker and the protective magnetic (磁的)field was switched on. Mars might be 【B8】______today, but life would have been possible in the past. 【B9】______
    But first we need to find a promising location, and that means reconstructing Martian history. 【B10】______, what type of minerals formed and if the geochemical conditions were compatible with life. Once promising sites are identified, we need to find out if there are 【B11】______
【B10】
选项
答案We need to find out if there was flowing water．the time and extent of the water
解析 find out，flowing water，time，extent，the water
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