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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 identify 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 word “flip” (Para．2) most probably means“ ______ ”.
选项 A、decline
B、intensify
C、fluctuate
D、reverse
答案D
解析 可从文中的“Although the strength of the magnetic field varies irregularly during the short terra，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．”推知，即虽然短期内磁场强度变化不规则，但似乎每次两极转换前都有一个不可避免的漫长衰落过程。当两极转换后——一个历时几十万年的过程，磁场迅速恢复其强度，循环便重新开始，因此reverse“颠倒，倒转”正确。
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