A、To contrast the reactions of babies to human and nonhuman sounds. B、To give examples of sounds that will cause a baby to cry.

Long before they can actually speak, babies pay special attention to the speech they hear around them. Within the first month of their lives, babies’ responses to the sound of the human voice will be different from their responses to other sorts of sounds. They will stop crying when they hear a person talking, but not if they hear a bell rings. At first, the sounds that an infant notices might be only those words that receive the heaviest emphasis and that often occur at the ends of sentences. By the time they are six or seven weeks old, babies can detect the difference between the rising and falling of tones. Very soon, these differences in adults’ stress and tones can influence babies’ emotional states and behavior. Long before they develop actual language comprehension, babies can sense when an adult is cheerful or angry, attempting to do or stop doing something, and so on, merely based on the rate, volume, and melody of adults’ speech.
    Adults make it as easy as they can for babies to pick up a language by exaggerating such characteristics. One researcher observed babies and their mothers in six diverse cultures and found that, in all six languages, the mothers used simplified grammar, short sentences, and transformed certain sounds into baby talk. Other investigators have found that when mothers talk to babies who are only a few months old, they often raise their volume, and speak slowly. They also exaggerate their facial expressions, and emphasize certain words.
    More significant for language development than their response to general intonation is observation that tiny babies can make relatively fine distinctions between speech sounds. In other words, babies enter the world with the ability to make precisely those perceptual discriminations that are necessary if they are to acquire aural language.
    Babies obviously derive pleasure from sound input, too: at even as young as nine months they will listen to songs or stories, although the words themselves are beyond their understanding. For babies, language is a sensory-motor delight rather than the route to dull meaning that it often is for adults.
    19.What does the talk mainly discuss?
    20.Why does the speaker mention the ringing of a bell?
    21.What can we learn from the findings of babies and their mothers in six diverse cultures?
    22.According to the talk, why do babies listen to songs and stories, even though they cannot understand them?