A、To illustrate the size of some objects. B、To introduce the next assignment. C、To show some artifacts on display at the campus

Today’s lecture will center on prehistoric people of the Nevada desert. Now, most of these prehistoric desert people moved across the countryside throughout the year. You might think that they were wandering aimlessly—far from it! They actually followed a series of carefully planned moves. Where they moved depended on where food was available—places where plants were ripening or fish were spawning.
    Now often when these people moved, they carried all their possessions on their backs, but if the journey was long, extra food and tools were sometimes stored in caves or beneath rocks. One of these caves is now an exciting archaeological site. Beyond its small opening is a huge underground grotto. Even though the cave’s very large, it was certainly too dark and dusty for the travelers to live in—but it was a great place to hide things, and tremendous amounts of food supplies and artifacts have been found there. The food includes dried fish, seeds, and nuts. The artifacts include stone spear points and knives: the spear points are actually rather small. Here’s a picture of some that were found. You can see their size in relation to the hands holding them.
5. What is the main subject of this talk?
6. What point does the speaker make about the prehistoric people of the Nevada desert?
7. Why didn’t people live in the cave described by the speaker?
8. What have archaeologists found in the cave?
9. Why does the speaker show a photo to the class?