A、To frighten other male alligators. B、To attract mates. C、To locate hatchlings. D、To threaten females. A由“This sends a strong，g

Narrator:
    Listen to part of a lecture in a Biology class.
Professor:
    OK. For today, let’s look at a reptile. No discussion of reptiles would be complete without some mention of crocodiles.
    Now, we tend to think of crocodiles as kind of mysterious creatures. But we are finding out that they aren’t as isolated as they seem. In fact, crocodiles interact with each other in a variety of ways. One way is with vocalizations, you know, sounds generated by the animal. This is true of the whole crocodile family, which includes crocodiles themselves, alligators, etc.
    Take American alligators. If you were to go to a swamp during the breeding season, you’d hear a chorus of sounds, deep grunts, hisses, these are sounds that male alligators make. And some of them are powerful enough to make the water vibrate. This sends a strong, go-away message to the other males. So the alligator can focus on sending other sound waves through the water, sound waves that you and I couldn’t even hear since they are at such low frequency. But they do reach the female alligator, who then goes to find and mate with the male.
    Vocalization is um...well, it is used for other reasons, like getting attention or just, urn...letting others know you are distressed. Let’s see. New-born crocodiles, or hatchlings and their interactions with their mothers. When they are born, baby crocodiles have a sort of muffled cry while they are in their nest. Hatchlings are really vulnerable, especially to birds and small mammals when they are born. But their mother, who has been keeping vigil nearby, hears their cry for help and carries them to safety, meaning, to water.
    So she takes them out of the nest. Uh, uh, all the eggs hatched at once, so she has about forty newborns to look after. Well, she takes about fifteen out of the nest at a time, carrying them in her mouth to the nearby water. While she is taking one load of hatchlings, the others wait for her to come back. But do you think they are quiet about it? No way. They are clamoring for the mother’s attention, sort of squeaking and practically saying—don’t forget about me!
    I heard some great examples of this on the television program on crocodiles last week. Anyone caught it? It had a few interesting bits. But you know, uh, you have to be careful, think critically. Sometimes I don’t know where these shows find their experts.
    Crocodiles’ brains are the most developed of any reptile. In that sense, they are closer to mammals’ brains than other reptiles’ brains. And we know that mammals, dogs for example, dogs vocalize many different sounds. Crocodiles have a similar level of, uh, vocal sophistication, if you will, which makes them unique among reptiles.
16. What does the professor mainly want to tell his students?
17. What are functions of the sounds made by male alligators?
18. What is one reason why hatchlings make vocalizations to their mother while they are in the nest?
19. What is the professor’s opinion about the television program that he mentions?