A、To hear a translation of her talk. B、To hear signals produced by electric fish. C、To hear sounds used to train electric fish.

This room is devoted to electric fish. The eel in the tank behind me can produce a strong jolt of electricity to stun its prey, but most of the fish in here produce only weak electrical impulses that are useful for navigating. Locating food, and even for communicating.
    The knife fish is good example. This fish navigates using tiny receptors in the skin that are sensitive to electrical impulses. The knife fish produces an electrical signal, and the receptors in its skin let it know when the signal it distorted by a tree root, or some other obstacle, so it can go around it.
    Fish also use the ability to produce and detect electrical impulses to communicate. They can tell each other what species they belong to, how big they are, and whether they’re male or female. We have a tank here that’s specially equipped to convert the inaudible signals the ish produce into sounds you can hear when you put on these headphones. I urge you all to listen in when I’m done speaking.
    Now have a look at the electric rays. Rays are especially interesting to medical researchers because of the organs they use to produce electricity. These organs contain a chemical that carries signals from one nerve ending to the next, not only in rays, but also in people. By studying these organs, scientists hope to learn more about diseases that interrupt the transmission of impulses from one nerve to another.
5. What is the talk mainly about?
6. What does the knife fish use electricity for?
7. Why does the speaker suggest putting on headphones?
8. According to the speaker, what medical benefits might from studying electric rays?