四级Passage 7 (2/2) - 英语听力
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When Stevenson followed the carbon-14 trail back in time, he found carbon-4 levels change with the intensity of solar burning. You see, the sun has cycles. Sometimes it burns fiercely, at other times it's relatively calm. During the sun's violent periods, it throws off charged particles in fast moving strings called solar winds. Here with the formation of carbon-14 on earth. When there's more solar wind activity, less carbon-14 is produced. Ten thousand years of tree rings show that the carbon-14 level rises and falls about every 420 years. The scientists concluded that the solar wind activity must follow the same cycle.
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