SECTION B PASSAGES
Directions: In this section, you will hear several passages. Listen to the passages carefully and then answer the questions that follow.
听力原文： There were several important discoveries in the early part of the century that helped 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 lava 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？

A. To review what students know about volcanic activity.
B. To demonstrate the use of a new measurement device.
C. To explain the answer to an examination question.
D. To provide background for the next reading assignment.