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  3. Now that we are fairly sure that there are many Earth-like planets in the Galaxy, the time is ripe (or almost so) to wonder whet

Now that we are fairly sure that there are many Earth-like planets in the Galaxy, the time is ripe (or almost so) to wonder whet

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问题     Now that we are fairly sure that there are many Earth-like planets in the Galaxy, the time is ripe (or almost so) to wonder whether these new worlds do indeed have a high probability of hosting new forms of life.
    Currently, the best【C1】______on the existence of other planets comes from NASA’s【C2】______Kepler satellite, which has been mapping an area of the sky【C3】______some 150,000 stars.
    Scientists estimate that around 5 percent of planets in our galaxy have masses similar【C4】______Earth’s and, possibly, are within the habitable zone of their stars, meaning that【C5】______they hold water it has a high chance of being liquid.【C6】______there are about 200 billion stars in our galaxy alone, scientists estimate that there should be about 10 billion Earth-size planets. Not bad, especially if we【C7】______that to be Earth-size and within a habitable zone is all it takes for life to take hold in an【C8】______world. In practice, however, the situation is much more【C9】______, since it depends on what life is and how it appears in a planet,【C10】______on the planet’s detailed geological history.
    Here on Earth life appeared some 3.5 billion years ago.【C11】______that it may have been here as far【C12】______as 3.8 billion years ago remain tentative. For about the first three billion years, life on earth was【C13】______in the form of single-cell creatures. Only when Earth’s atmosphere was full of oxygen did more sophisticated organisms appear.
    【C14】______its unusual atmosphere, Earth’s heavy, single moon, its magnetic field also did something else that allowed complex life to【C15】______on its surface.【C16】______these, Complex life would find it very hard to【C17】______and survive.
    The results from Kepler’s mission are encouraging and exciting. We have the【C18】______of confirming the existence of countless other worlds out there. Based on these findings, we should indeed【C19】______that other worlds would host some kind of simple life. However, as some scientists argued, Earth and its ability to host complex life will probably remain a【C20】______phenomenon in this and other galaxies.
【C6】
选项 A、Provided
B、Hence
C、Since
D、Unless
答案C
解析 结合本段首句中提到的“科学家估计约有5％的行星具有跟地球相似的质量”，可以认为此句逗号前的分句“单单在银河系中就存在大约2,000亿颗恒星”是逗号后句子“科学家们估计大约会有100亿颗地球体积大小的行星存在”的依据。C项Since可引导原因状语从句，指“由于，因为”，用法、语义均符合要求。
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