自2013年12月考次起，全国大学英语四、六级考试委员会将对四、六级考试的试卷结构和测试题型作局部调整。快来一睹四六级新题型的风采吧~
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.