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第一段
1 .Now listen to part of a lecture on the topic you just read about.
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第二段
1 .Unfortunately, we still don't know what's killing the yellow cedar, none of the explanations discussed in the reading is adequate.
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第三段
1 .First, the cedar bark beetle.
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2 .Well, the problem with this explanation is that healthy yellow cedars are generally much more resistant to insect infestation than other tree species.
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3 .For example, the bark and leaves of the yellow cedar are saturated with powerful chemicals that are poisonous to insects.
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4 .So, healthy cedars are unlikely to suffer from insect damage.
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5 .So, how can we explain those dead cedars that were infested with beetles?
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6 .In those cases, the beetles attack trees that were already damaged or sick, and would've probably died anyway.
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7 .So, the beetles are not the fundamental cause responsible for the decline of the yellow cedars.
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第四段
1 .Second, although bears damage some trees, there're not the cause of the overall population decline.
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2 .Yellow cedar population's been declining all across the northwestern coast of North America both on the mainland and on islands just off the coast.
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3 .There are no bears on the islands, yet the islands'cedars are still in decline.
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4 .Since the decline occurs with and without bears, the bears cannot be responsible.
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第五段
1 .And finally, the theory about the roots suffering from frost damage, well, the reading passage forgot to take one fact into account.
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2 .Many more trees are dying at lower elevations where it is warm than at higher elevations where it is cold.
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3 .If freezing damage were responsible for the decline, we would expect to see more trees dying in the cold weather of high elevations.
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4 .Instead, more trees are dying in the relative warmth of the low elevations.
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5 .So, although the climate change may have made the cedar roots more sensitive than they used to be, this isn't what's killing them.
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