听力题 You can tell the age of a tree by counting 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 surface for the last ten thousand years. Each ring represents a year of growth. As a tree grows, it adds a layer to its trunk, taking up chemical dements from the air. By looking at the elements in the tings from a given year, scientists can tell what elements were in the air that year. Doctor Stevenson is analyzing 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 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 streams called "solar winds". The particles interfere 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 solar wind activity must follow the same cycle.