单选题 For an infant just beginning to interact with the surrounding world, it is imperative that he quickly become proficient in his native language. While developing a vocabulary and the ability to communicate using it are obviously important steps in this process, an infant must first be able to learn from the various streams of audible communication around him. To that end, during the course of even the first few months of development, an infant will begin to absorb the rhythmic patterns and sequences of sounds that characterize his language, and will begin to differentiate between the meanings of various pitch and stress changes. However, it is important to recognize that such learning does not take place in a vacuum. Infants must confront these language acquisition challenges in an environment where, quite frequently, several streams of communication or noise are occurring simultaneously. In other words, infants must not only learn how to segment individual speech streams into their component words, but they must also be able to distinguish between concurrent streams of sound. Consider, for example, an infant being spoken to by his mother. Before he can leam from the slight differences of his mother"s speech, he must first separate that speech from the sounds of the dishwasher, the family dog, the bus stopping on the street outside, and, quite possibly, background noise in the form of speech; a newscaster on the television down the hall or siblings playing in an adjacent room. How exactly do infants wade through such a murky conglomeration of audible stimuli? While most infants are capable of separating out two different voices despite the presence of additional, competing streams of sound, this capability is predicated upon several specific conditions. First, infants are better able to learn from a particular speech stream when that voice is louder than any of. the competing streams of background speech; when two voices are of equal amplitude, infants typically demonstrate little preference towards one stream or the other. Most likely, equally loud competing voice streams, for the infant, become combined into a single stream that necessarily contains unfamiliar patterns and sounds that can quite easily induce confusion. Secondly, an infant is more likely to attend to a particular voice stream if it is perceived as more familiar than another stream. When an infant, for example, is presented with a voice stream spoken by his mother and a background stream delivered by an unfamiliar voice, usually he can easily separate out her voice from the distraction of the background stream. By using these simple yet important cues an infant can become quite adept at concentrating on a single stream of communication and, therefore, capable of more quickly learning the invaluable characteristics and rules of his native language.

单选题 Which of the following best conveys the main idea of Paragraph 1 ?

单选题 The phrase "predicated upon several specific conditions"(Para. 4)is used by the author to suggest that______.

单选题 The author uses the word "necessarily"(Line 4 of Para. 5)in order to suggest that______.

单选题 Before an infant can learn from the slight differences of his mother"s speech, he must first______.

单选题 The example in the last paragraph is used to illustrate how______.