Lecture的结构是帮助我们理解文章逻辑的利器，而能够将复杂的Lecture分层，听出结构则是一个重要的能力。本篇Lecture是非常典型的现象解释型文章，而且它的结构和每题考点都是紧密相关的。
文章结构如下：
引出主题：开篇描述大型动物的灭绝(all those giant creatures, the mega-fauna of North America were gone.)来引出下面通过几个理论来解释。(We don't know exactly what happened to them, but there are some theories.) ------对应23题
Hunting theory. 需要听到记到的内容就是这个theory 描述的解释是什么以及教授这个解释的反驳。
解释：One theory is that they were hunted to extinction by humans. 要知道这个human文中具体指的就是Clovis People. 紧接着也对这个theory进行了一定支持。 ------对应24题
教授反驳：从听到But then….开始说明这个theory还是有问题的，教授直接表达了他的观点：So I think it is quite likely that those thirteen mammoths in southern California also died of natural causes, and that the Clovis people simply took advantage of the situation. ------对应25题
Climate change theory. 和hunting theory 需要抓住的重点信息和climate theory 相同。
这个解释有二：1. These extreme shifts would have put a lot of stress on the bodies of animals that were used to a more moderate range of temperatures
2. But the most important impact of this increased seasonality may very well have been its effect on the distribution of plants. And the great beasts were the ones that needed the most diversity in their diet. ---------对应双选26题
教授反驳：but then, how do you explain the fact that this has happened before? You know, global cooling followed by global warming, and there was no extinction then.------对应27题
原来正解需要computer的帮助，结论依然有两个方面：
1.One thing….soall you have to do is remove a few young females from the herd, and you can, fairly quickly, significantly reduce the population
2. And then he came up with a scenario that combined some hunting by humans with some environmental stress, and...Bang! The simulated mammoths were extinct within decades.------对应双选28题
听准红色信号词可以有助于理解文章的结构。当然听结构的能力也不是一下子就能形成的，需要坚持对文章的结构进行分析。