[单项选择]The collapse of the Earth’s magnetic field—which guards the planet and guides many of its creatures—appears to have started seriously about 150 years ago, the New York Times reported last week.
The field’s strength has decreased by 10 or 15 per cent so far and this has increased the debate over whether it signals a reversal of the planet’s lines of magnetic force.
During a reversal, the main field weakens, almost vanishes, and reappears with opposite polarity. The transition would take thousands of years. Once completed, compass needles that had pointed north would point south. A reversal could cause problems for both man and animals. Astronauts and satellites would have difficulties. Birds, fish and animals that rely on the magnetic field for navigation would find migration confusing. But experts said the effects would not be a big disaster, despite claims of doom and vague evidence of links between past field reversals and species extinctions.
Although a total transiti
A. destroy almost all the creatures on the Earth
B. cause some species extinctions on the Earth
C. not be as disastrous as the previous one
D. cause no big trouble for man and animals

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