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听力原文： You can tell the age of a tree by counting its rings, but these records of trees’ life really say a lot more. Scientists are using tree tings 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, it adds a layer to its trunk taking up chemical elements from the air. 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 analyzing one element—carbon-14 in ring from both living and dead trees. Some of the tings go back almost ten thousand years to the end of the Ice Age. When Stevenson followed the carben-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, it throws off charged particles in fast moving strings called solar winds. The particles interfere with the formation of carben-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 can the tree's rings tell us？
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听力原文：W: Well, Kevin, as a college graduate, how did you become a taxi driver？
M: Well, you do it for money, obviously, like most jobs, but I enjoyed it. I suppose I enjoy being a taxi driver, (19) because 1 can be my own boss, you know, doing what I want to do. You decide what area you're going to, when to have a break, stuff like that. (20)One thing you have to be always thinking about is where you may pick up a passenger. You're always sort of scheming to make an exact bit of money. And the relationship between reward and effort is very immediate. You make the right decision, they pay you and you get a fare.
W: How do the passenger feel when you talk to them？
M: You meet some people who aren't all that nice, (21)but the vast majority of people are very nice. If you're pleased to talk to them, they're mostly pleased to talk to you. You get a feel for people who don't want to talk to you. Ob-viously the first couple of one-word answers will tell you.
W: So you always start talking？
M: I always wish people "good morning" or "good afternoon" or whatever that gets things off to a good start, because quite a few people have a general dislike of taxi drivers.
W: Do they？
M: Well, they do. I mean, the mere fact they can't get a cab the minute they want one makes them annoyed.
(20)
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