【正确答案】 B

【答案解析】[听力原文] You can tell the age of a tree by counting its rings. But these records of 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 elements from the air. By looking up the elements in the rings from the given year, scientists can tell what elements were in the air that year. Dr. 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 and other times it is relatively calm. During the sun's violent periods, it throws off charged particles in fast moving strings 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 the solar wind activity must follow the same cycle. What is the purpose of the scientists in studying tree rings? 细节辨认题。短文第二句指出Scientists are using tree rings to learn what's been happening on the sun's surface for the last ten thousand years，故科学家研究树年轮的目的是为了调查在过去一万年前太阳表面到底发生了什么。所以选项B正确。

【正确答案】 C

【答案解析】[听力原文] What affects the amount of carbon-14 on earth? 细节辨认题。短文中指出he found carbon-14 levels change with the intensity of solar burning，故地球上碳14的含量受太阳燃烧强度的影响。所以选C。

【正确答案】 D

【答案解析】[听力原文] What do we learn from the passage about the solar wind activity? 事实细节题。根据短文最后一句The scientists concluded that the solar wind activity must follow the same cycle，可知太阳风的活动遵循着一个相同的周期。所以选D。