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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:->Quackers are certainly a very strange phenomenon.
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2 .Experts still debate what the source of the sounds was.
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3 .No one can be sure exactly what caused them, but these experts cite certain problems with all of the theories that you just read about.
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4 .Here are a few of the arguments that they make.
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第3段
1 .First, the idea that the sounds were 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, but orca whales mostly live near the surface of the water.
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3 .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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4 .Also, the orca whales would have been detected by the Russian sonar if they were nearby.
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第4段
1 .Giant squid may be a better candidate, but one critical fact speaks against the squid theory as well: Russian submarines first detected quacker sounds in the 1960s, and reports of them continued for about two decades, but the sounds disappeared entirely by the 1980s.
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2 .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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3 .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5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, but no such noise accompanied the quackers.
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5 .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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