A、The lifecycle of trees. B、The number of trees. C、The intensity of solar burning. D、The quality of air. C细节题。短文中明确提到，Stevenson发

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.[16]Scientists are using tree rings to learn what’s been happening on the sun’s surface for the last 10,000 years. Each ring represents a year of growth. As the tree grows, it adds its layer to its trunk, taking up chemical 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. Dr. Stevenson is analysing one element, carbon 14 in rings from both living and dead trees. Some of the rings go back almost 10,000 years to the end of the Ice Age.[17]When Stevenson followed the carbon 14 trail back in time he found carbon 14 levels change with the intensity of solar burning.[18]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. The particles interfere with the formation of carbon 14 on earth. When there’s more solar wind activity, less carbon 14 is produced. And 10,000 years of tree rings show that the carbon 14 level rises and falls about every 420 years. But scientists concluded that solar wind activity must follow the same cycle.
16. What’s the purpose of the scientists in studying tree rings?
17. What affects the amount of carbon 14 on Earth?
18. What do we learn from the passage about solar wind activity?