What does this passage tell us? It tells us why our brains______.

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问题 What does this passage tell us? It tells us why our brains______.
  
Human brains are about three times as large as those of our early ancestors that lived four million to two million years ago, and for years, (11)scientists have wondered how our brains got so big. (12) A new study suggests social competition could be behind the increase in brain size.
    Compared to almost all other animals, human brains are larger as a percentage of body weight. (13) And since the emergence of the first species about two million years ago, the human brain has doubled in size. And when compared to earlier ancestors, (14)such as those that lived four million to two million years ago, our brains are three times as large. For years, scientists have wondered what could account for this increase.
    The three major theories have focused on climate change, the demand of ecology, and social competition.

选项

答案three times

解析 本题询问与生活在四百万年至二百万年前的早期祖先相比,现在人类的大脑有多大。根据such as those that lived four million to two million years ago,our brains are three times as large可知,与那些生活在四百万年至二百万年前的早期人类相比,现在人类的大脑是他们的三倍大。
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