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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 .Quackers are certainly a very strange phenomenon.
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2 .Experts still debate what the source of the sound was.
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3 .No one can be sure exactly what caused them.
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4 .But these experts cite certain problems with all of the theories that you just read about.
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5 .Here are a few of the arguments that they make.
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第三段
1 .First, the idea that the sounds are caused by Orca whales seems plausible at first but is ultimately highly unlikely.
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2 .It's true there were Orca populations in the general areas that the Russian submarines were patrolling.
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3 .But Orca whales mostly live near the surface of the water.
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4 .The submarines typically remained deep in the ocean and should not have been able to hear the whale sounds from near the surface.
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5 .Also, the Orca whales would have been detected by the Russian sonar if they were nearby.
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第四段
1 .Giant squid may be a better candidate but one critical fact speaks against the squid theory as well.
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2 .Russian submarines first detected quacker sounds in the 1960s and reports of them continued for about two decades.
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3 .But the sounds disappeared entirely by the 1980s.
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4 .However, as far as we know, squid have always lived in the ocean where the submarines were patrolling and continue to live there today.
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5 .If these were squid sounds, there would be no reason to suddenly start hearing them in one decade and then suddenly stop hearing them twenty years later.
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第五段
1 .Third, the idea that the quackers were caused by a secret submarine from another country does not hold up.
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2 .The sources of the sounds appeared to move around and change direction very quickly.
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3 .Submarines cannot move or change direction that quickly.
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4 .Also, all submarines make some engine noise.
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5 .But no such noise accompanied the quackers.
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6 .Even today, we don't have technology to build submarines that are that fast and have engines that are that silent.
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7 .Summarize the points made in the lecture,being sure to explain how they challenge the specific theories presented in the reading passage.
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