A、Relics of human bodies. B、Bones. C、Food and tools. D、Fish and nuts. C推断题。根据对话中But if the journey was long，extra food and tools

Today’ s lecture we’ll centre on the people who lived five thousand years ago in the Sahara Desert. Now most of these desert people moved across the countryside throughout the year. You might think that they’re wandering aimlessly. Far from it, they actually followed the series of carefully planned moves.
    Where they moved depended on where food was available, places where plants were ripening or with lots offish. 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.
    One of these caves is now an exciting historic site. Even though the cave is very large, it was certainly too dark and dusty for people to live in, but it was a great place to hide things. And huge amounts of food supplies and daily tools have been found there.
    The food includes dried fish and nuts. The tools include stone spear points and knives. The spear points are actually rather small. Here is a picture of some that were found. You can see their size in relation to the hands holding them.
    There are also some decorations found in the caves. There are necklaces, ear rings, etc. Most of them were made of bones, instead of some rare metals like what those are today. We may not like the idea of it. But it was very popular at that time. From the above talk, we can see there ’re really great differences from our lives today.
Questions 16 to 18 are based on the passage you have just heard.
16. What is the main subject of this talk?
17. What have been found in the cave?
18. Why does the speaker show a photo to the class?