单选题

A It affects the growth of trees.

B It has been increasing since the Ice Age.

C It is determined by the chemicals in the air.

D It follows a certain cycle.

解析：
细节题。录音明确提到“the sun has cycles”，最后说明“The scientists concluded that solar wind activity must follow the same cycle.”可推断“Solar wind activity follows a certain cycle”。故选D。
【录音原文】
　　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 10,000 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 at the elements in the rings 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 10,000 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.
Questions 1 to 3 are based on the passage you have just heard.
1. What’s the purpose of the scientists in studying tree rings?
2. What affects the amount of carbon 14 on Earth?
3. What do we learn from the passage about solar wind activity?