2007英语四级听力综合指导(四十七)
Passage Three
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 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 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 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 is more solar wind activity, less carbon-14 is produced. 10,000 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.
18. What's the purpose of the scientists in studying tree rings?
19. What affects the amount of carbon-14 on Earth?
20. What do we learn from the passage about solar wind activity?
18.
A) To examine the chemical elements in the Ice Age.
B) To look into the pattern of solar wind activity.
C) To analyze the composition of different trees.
D) To find out the origin of carbon-14 on Earth.
19.
A) The lifecycle of trees.
B) The number of trees.
C) The intensity of solar burning.
D) The quality of air.
20.
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.
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