A、Tropical insects. B、Decayed leaves. C、Tree resin. D、Bird feathers. C细节题。题目问琥珀是如何形成的，根据Amber is really fossilized tree resin．可知

I’m going to pass this piece of amber around so you can see this spider trapped inside it. It’s a good example of amber-inclusion, one of the inclusions that scientists are interested in these days. This particular piece is estimated to be about 20 million years old. Please be extremely careful not to drop it. Amber shatters as easily as glass. One thing I really like about amber is its beautiful golden color. Now, how does the spider get in there? Amber is really fossilized tree resin. Lots of chunks of amber contain insects like this one or animal parts like feathers or even plants. Here is how it happens. The resin oozes out of the tree and the spider or leaf gets in cased in it. Over millions and millions of years, the resin hardens and fossilizes into the precious stone you see here. Ambers can be found in many different places around the world. But the oldest deposits are right here in the United States, in Appalachia. It’ s found in several other countries, too, though right now scientists are most interested in ambers coming from the Dominican Republic. Because it has a great many inclusions, something likes one insect inclusion for every one hundred pieces. One possible explanation for this it that the climate is tropical and a greater variety of number of insects thrive in tropics than in other places. What’s really interesting is the scientists are now able to recover DNA from these fossils and study the genetic material for important clues to revolution.
Now listen to the following recording and answer questions 16 to 19.
16. Why does the professor pass the amber around to the students?
17. What is amber derived from?
18. Why is the Dominican Republic an important source of amber?
19. What type of amber is probably the most valuable for genetic research?