You can tell the age of a tree by【B1】______ its rings, but these records of

游客2023-07-06 22

问题 You can tell the age of a tree by【B1】______ its rings, but these records of a tree’s life really say a lot more. Scientists are using tree rings to learn what’s been happening on the sun’s【B2】______ for the last ten thousand years. Each ring【B3】______a year of growth. As a tree grows, it adds a layer to its trunk, taking up【B4】______ elements from the air. By looking at the elements in the rings from a given year, scientists can tell what elements were in the air that year. Doctor Stevenson is【B5】______one element—carbon-14, in rings from both living and dead trees. Some of the rings go back almost ten thousand years to the end of the Ice Age. When Stevenson followed the carbon-14 trail back in time, he found carbon-14 levels change with the【B6】______ of solar burning. You see the sun has cycles. Sometimes it burns【B7】______ at other times it’s relatively calm. During the sun’s【B8】______ periods, it 【B9】______ charged particles in fast moving streams called "solar winds". The particles【B10】______ 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 solar wind activity must follow the same cycle. [br] 【B4】

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