六级练习2015.06.06
[audio]http://sound.yywz123.com/tingbbs/cet6/cet620150606.mp3[/audio]You can tell the age of a tree by counting its rings (1)_________________. Scientists are using tree rings to learn what’s being happening on the sun’s surface for the last ten thousand years. Each ring represents a year of growth. As the tree grows, (2)___________________. By looking up the elements in the rings for a given year scientists can tell what elements were in the air that year. Doctors Stevenson is analysing one element carbon-14 in ring 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-4 levels change with the intensity of solar burning. You see the sun has cycles. Sometimes it burns fiercely and other times it’s relatively calm. During the sun’s violent periods (3)___________________. 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.
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