[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. regularly changes its strength and axis
B. is relatively more difficult to conduct research on
C. is dynamic and constantly changing
D. is usually analyzed by deep sea sediment

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
Groundbreaking research by two French geophysicists promises to shed some light on the mystery. Using 80 metres of deep sea sediment (沉淀物) 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 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 magnet
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

There are some steps you can take yourself to avoid catching a cold. Contrary to popular belief, colds are not caused by exposure to severe weather. Colds are caused by viruses (病毒) harbored in the body, and you’re better off out on the ski slopes or even waiting for the bus on a snowy day than you are in a warm room, surrounded by friends, co-workers, or fellow students, who just may be passing the viruses around. If you feel a chill when you’re coming down with a cold, you’re already sick. A chill is an early symptom: it’s the cold that caused the chill, not the other way around.
While the virus can spread through droplets (飞沫) propelled into the air when a cold-sufferer coughs or sneezes. Surprisingly, this is not the most common route of transmission, Numerous studies have now shown that the overwhelming majority of colds are "caught" by hand contact, A cold sufferer rubs her nose, thereby transferring the virus to her hand. Then a fr
A. attend a party with a lot of people
B. stay long hours in the street on a snowy day
C. go out on an extremely cold and windy day
D. go skiing or skating in the open air