[单项选择]There are some earth phenomena you can count on, but the magnetic field, some say, is not one of them. It fluctuates in strength, drifts from its axis, and every few 100,000 years undergoes 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
Research by two French geophysicists promises to shed some light on the mystery. Using 80 meters of deep sea sediment (沉淀物), 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 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 strengt
A. Polarity Reversal: A Fantastic Phenomenon of Nature.
B. Measurement of the Earth’s Magnetic-Field Intensity.
C. Formation of the Two Poles of the Earth.
D. A New Approach to the Study of Geophysics.

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