【正确答案】 D

【答案解析】[听力原文]
   I'm glad you brought up the question of our investigations into the makeup of the earth's interior. In fact, since this is the topic of your reading assignment for next time, let me spend these last few minutes of class talking about that.
   There were several important discoveries in the early part of the century to help geologists develop a more accurate picture of the earth's interior. The first key discovery had to do with seismic waves. Remember, they are the vibrations caused by earthquakes. Well, scientists found that they travel thousands of miles through the earth's interior. This finding enabled geologists to study the inner parts of the earth.
   You see, these studies revealed that these vibrations were of two types: compression or P waves and shear or S waves. And researchers found that P waves travel through both liquids and solids while S waves travel only through solid matter. In 1906, a British geologist discovered that P waves slowed down at a certain depth but kept traveling deeper. On the other hand, S waves either disappeared or were reflected back. So he concluded that the depth marked the boundary between a solid mantle and the liquid core. Three years later, another boundary was discovered, that between the mantle and the earth's crust.
   There are still a lot to be learned about the earth. For instance, geologists know that the core is hot. Evidence of this is the molten rock that flows out of volcanoes. But we are still not sure what the source of the heat is.
   What is the purpose of the talk?
快速浏览四个题目的所有选项，由其中的关键词students、assignment、Earth's interior、molten rock等可大致推断，文章讲的内容很可能与学生有关，而且可能围绕地球内部结构这一问题展开，因此需要重点关注相关内容。
   1.四项均为不定式短语。
   2.从选项内容看都表示目的。
   结论：本题考查目的，很可能提问文中某一行为的目的或短文的主旨。
   录音开头说话人表示，很高兴学生提出了关于地球内部结构研究的问题，因为这是下次阅读作业的主题，因此有必要花几分钟时间谈论一下。可见短文的主要内容是关于地球内部结构研究的背景知识，故选D。