自2013年12月考次起，全國(guó)大學(xué)英語(yǔ)四、六級(jí)考試委員會(huì)將對(duì)四、六級(jí)考試的試卷結(jié)構(gòu)和測(cè)試題型作局部調(diào)整?？靵?lái)一睹四六級(jí)新題型的風(fēng)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.