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一般来说，所谓“原汁原味”的自然对话中，适合外语学习的“精华”含量会比较少。如果希望文字中出现相关单词的联用形式的比例比较高，那么对学习内容不但要限制难度，还要规范内容。建议选择词汇关联度高的、紧密围绕一个概念或主题展开的“主题内容”（thematic context）。无论是学校使用的外语教材，还是市场上流行的国际知名的经典外语教材（比如《新概念》《走遍美国》等），都包含了经过加工的“主题内容”，非常不错。然而此时如果仅局限于使用一套教材，最严重的问题就是某一难度下的材料内容数量有限，往往都是短短几篇课文就转到其他难度和内容上去了。所以一些学生很辛苦地集中啃一套教材，造成在每个阶段的输入量都远远不够。而再拿起另一套教材时，很多难度又已经不适合当前学习了。
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学单词，就是要对同一个主题反复啰嗦效果才好。
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所以如果希望锁定一套学习材料又要求其中含有大量的“主题内容”，那么无论从难度到内容，从质量到数量，一个典型的非常适合学习的最佳学习材料样板，又要说是《美国之音》英语学习节目。从内容主题的角度来说，《美国之音》英语教学中包括了科技、时事、教育、社会、人文和经济等非常丰富的各种主题版块，比如：Technology Report, This is America, Agriculture Report, Science in the News, Health Report, Explorations, Education Report, The Making of a Nation, Economics Report, American Mosaic, In the News, American Stories, People in America等等。
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比如下面《美国之音》英语学习节目科学报道中有关“地震”主题的内容：
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Scientists Predicting Strength of Future Earthquakes
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Scientists know where earthquakes are likely to take place. But it is difficult for them to predict how strong an earthquake will be.Now, American researchers have found that it may be possible to predict the strength of future quakes.
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The Earth’s crust—the outermost surface—is made up of large tectonic plates. As many as 20 tectonic plates cover the Earth.They move slowly, sometimes sliding under one another.Where that happens, earthquakes can take place.
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Timothy H. Dixon is with the University of South Florida.He is a geologist—a scientist who studies rocks and soil to understand the Earth.He and other scientists are using global positioning system, or GPS, equipment to measure what he calls”slow-slip”events.By that, he means slow-moving earthquakes that cannot be measured by other instruments.
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Timothy Dixon says that while such movements may not be felt, they can lead to large, sometimes deadly earthquakes.“They’re the kind of earthquake that made the big Japan earthquake in 2011 and the Sumatra earthquake and tsunami in 2004. So they can be, they can be the real killers.So what we think these slow-slip events are telling us is, they’re letting off steam, so to speak, in areas of the fault.And if they let off enough steam, that part of the fault won’t rupture.“The more of the fault that ruptures, or breaks, he says, the more powerful the earthquake is.
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The scientists set up a number of GPS receivers 15 years ago in Costa Rica to measure the”slow-slip”events, which take place only every year or two. The last one was three months before an earthquake in 2012.
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People always want to know when an earthquake is going to happen. This technology cannot predict that.But Timothy Dixon says knowing how strong the quake might be gives officials time to improve building codes and make other preparations.
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“You know the earthquake is more-or-less inevitable. Even if you don’t know exactly when it’s going to happen, since you know it’s going to happen, and you know how big it’s going to be, you can prepare accordingly.”
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A scientific paper presenting his findings was published in the Proceedings of the National Academy of Sciences.
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Ashley Thompson wrote it for VOA Learning English.December,2014
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上面一段中，各种与核心主题相关的词汇连接和与生词的搭配都出现在什么地方呢？
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●earthquakes take place strong/strength difficult to predict inevitable
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●measured by instruments global positioning system（GPS）
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●geologist studies rocks and soil slow slip fault rupture
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●earth’s crust outmost surface tectonic plates sliding under one another
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●movements be felt large deadly earthquakes tsunami
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●scientific paper presenting his findings published in
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还有几个俗语：more-or-less;letting off steam;so to speak
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最理想的学习状态，是使用多个具有同样主题内容的材料，让同样的词汇和词汇搭配以稍微不同的形式再次出现，这样会令印象更加深刻。我们再找一篇《美国地理调查》网站上的地震灾害项目专题，其中有很多对儿童和青少年进行科普的文章。比如下面这篇文章：
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