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第1段
1 .<-NARRATOR:->Now listen to part of a lecture on the topic you just read about.
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第2段
1 .<-FEMALE PROFESSOR:->While the theories given in the reading may sound plausible, none of them is a good explanation for the mass extinction at the end of the Triassic period.
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第3段
1 .First, sea level change. While scientists agree that the sea level fluctuated at the end of the Triassic period, often going down, this isn’t a good explanation for the extinctions.
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2 .Coastal and shallow-water ecosystems are usually capable of adapting to environmental changes that happen gradually.
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3 .The fall in sea level at the end of the Triassic was quite gradual, taking place over several million years.
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4 .The change would have to be much more sudden to have a widespread negative impact on the species in those ecosystems.
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第4段
1 .Second, global cooling. It’s true that sulfur dioxide can lower global temperatures.
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2 .But that can only happen during a relatively short period when the sulfur dioxide that’s been released by volcanoes is actually still present in the atmosphere.
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3 .In a matter of a few years, the excess SO2 is usually cleared out of the atmosphere.
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4 .Basically, the SO2 combines with water in the atmosphere and falls back on Earth as rain.
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5 .It doesn’t seem likely, therefore, that even if there was a lot of volcanic SO2 released at the end of the Triassic, it stayed in the atmosphere long enough to cause mass extinctions.
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第5段
1 .Third, very few scientists believe the asteroid theory because we haven’t found any asteroid crater—the site where the asteroid hit—that can be dated to the time when the mass extinction occurred.
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2 .We did find a crater, but it dates to about twelve million years before the extinction.
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3 .That’s just too long before the extinction to have anything to do with it.
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