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  3. There are some earth phenomena you can count on, but the magnetic field, someday is not of them. It fluctuates in strength, drif

There are some earth phenomena you can count on, but the magnetic field, someday is not of them. It fluctuates in strength, drif

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问题     There are some earth phenomena you can count on, but the magnetic field, someday is not of them. It fluctuates in strength, drifts from its axis, and every few 100,000 years undergo a dramatic polarity reversal—a period when north pole becomes south pole and south pole becomes north pole. But how is the field generated, and why is it so unstable?
    Groundbreaking research by two French geophysicists promises to shed some light on the mystery. Using 80 meters of deep sea sediment core, they have obtained measurements of magnetic-field intensity that span 11 polarity reversals and four million years. The analysis reveals that intensity appears to fluctuate with a clear, well-defined rhythm. Although the strength of the magnetic field varies irregularly during the shout term, there seems to be an inevitable long term decline preceding each polarity reversal. When the poles flip—a process that takes several hundred thousand years—the magnetic field rapidly regains its strength and the cycle is repeated.
    The results have caused a stir among geophysicists. The magnetic field is thought to originate from molten iron in the outer core, 3,000 kilometers beneath the earth’s surface. By studying mineral grains found in material ranging from rocks to clay articles, previous researchers have already been able to i-dentify reversals dating back 170 million years, including the most recent switch 730,000 years age. How and why they occur, however, has been widely debated. Several theories link polarity flips to external disasters such as meteor impacts. But Peter Olson, a geophysicist at the Johns Hopkins University in Baltimore, says this is unlikely if the French researchers are right. In fact, Olson says intensity that predictably declines from one reversal to the next contradicts 90 percent of the models currently under study. If the results prove to be valid, geophysicists will have a new theory to guide them in their quest to understand the earth’s inner physics. It certainly points the direction for future research.
The French geophysicists’ study is different from currently prevailing theories in_____.
选项 A、identification of the origin of the earth’ s magnetic field
B、the way the earth’s magnetic intensity is measured
C、explanation of the shift in the earth’s polarity
D、the way the earth’s fluctuation rhythm is defined
答案A
解析 题的四个选项中，只有A项为正确答案，这可从文中第三段的内容推知，即这些结果引起了地球物理学家们的震惊，人们认为磁场源于地表下3000公里处地核外层熔化的铁。后面又说，在巴尔的摩的约翰斯-霍布金斯大学的地球物理学家彼得·奥尔森说，如果法国研究者正确的话，这几种理论就不可能成立了。奥尔森说，实际上两次转换问预计的衰落强度同90％的现行研究模式相矛盾。如果这些结果证明正确，地球物理学家将会有一种新理论来指导他们探索地球内部物理学。它的确为将来的研究指出了方向。
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