再问：L16-3-24

weiyu · 发表于 2004-2-28 14:23:00

Question24
When    volcanic    lava    solidifies,    it    becomes    uniformly    magnetized    in    the    direction    in    which    the    Earth's    magnetic    field    points.    There    are    significant    differences    in    the    direction    of    magnetization    among    solidified    lava    flows    from    different    volcanoes    that    erupted    at    different    times    over    the    past    several    million    years.    Therefore,    it    must    be    that    the    direction    of    the    Earth's    magnetic    field    has    changed    over    time.    Since    lava    flows    differing    by    thousands    of    years    in    age    often    have    very    similar    directions    of    magnetization,    the    change    in    the    direction    of    the    Earth's    magnetic    field    must    take    place    very    gradually    over    hundreds    of    thousands    of    years.
24.    The    argument    that    the    direction    of    the    Earth's    magnetic    field    has    changed    over    time    requires    the    assumption    that
(A)    only    lava    can    be    used    to    measure    the    direction    of    the    Earth's    magnetic    field    as    it    existed    in    the    distant    past
(B)    a    single    volcano    can    produce    lava    of    differing    consistencies    during    different    eruptions
(C)    not    all    solidified    lava    has    changed    the    direction    of    its    magnetization    unpredictably
(D)    there    are    fewer    volcanic    eruptions    now    than    there    were    millions    of    years    ago
(E)    as    lava    flows    down    the    side    of    a    volcano,    it    picks    up    magnetized    rocks

Explanation of Cranberry host: 大前提是岩浆磁场和地球的磁场走向一致，小前提是许多岩浆的走向不一致，结论是地球的磁场走向变化了。我们测量岩浆的磁场是现在，这就要求在这许多年里，岩浆的磁场没有变化，或没有无法预期的变化。

我的问题：但是原文明明说了lava    flows    differing    by    thousands    of    years    in    age    often    have    very    similar    directions    of    magnetization,    the    change    in    the    direction    of    the    Earth's    magnetic    field    must    take    place    very    gradually    over    hundreds    of    thousands    of    years.    这些lava flows方向不同，但磁场方向相同，C的假设说明什么呢？

cranberry · 发表于 2004-2-28 17:42:00

你对原文的中文意思理解的不对，其含义是，年代相差千万年的岩浆流有相同的磁场走向，说明地球磁场是渐变的。

jnlvo · 发表于 2004-2-28 22:24:00

我认为since后对问题的argument无用，看since前。c项取反就是all solidified lava has changed the direction of its magnetization unpredictably 否定了原文的前提条件the direction of lave's magnetization和earth's一致。

再问：L16-3-24

再问：L16-3-24

所属分类: GMAT考试

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