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

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

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问题     There are some 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 100000 years undergo, 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(沉淀物)core, they have obtained measurements lots 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 short 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 (熔化的)iron in the outer cote, 3000 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 identify reversals dating back 170 million years, including the most recent switch 730, 000 years ago. How and why they occur, however, has been widely debated. Several theories link polarity flips to external disasters such as meteor(陨星) 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、its identification of the origin of the earth’s magnetic field
B、the way the earth’s magnetic intensity is measured
C、its explanation of the shift in the earth’s polarity
D、the way the earth’s fluctuation rhythm is defined
答案C
解析 文章表明，物理学家用不同的方法解释地球极性的转变。这两位物理学家用80米以下的深海沉积物做标本，而其他物理学家研究岩石里的矿物颗粒，以及有人认为是外部力量诸如陨星侵害来解释。第二段Using 80 meters of deep sea sediment(沉淀物)core，第三段By studying miner-al grains found in material ranging from rocks to clay articles．Several theories link polarity flips toexternal disasters such as meteor(陨星)impacts．
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