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　　Text 2

　　If you live in a large city, you are quite familiar with some of the problems of noise, but because of some of its harmful effects, you may not be aware of the extent of its influence on human behavior. Although everyone more or less knows what noise is, i.e., it is sounds that one would rather not hear, it is perhaps best to define it more precisely for scientific purposes. One such definition is that noise is sounds that are unrelated to the task at hand. Thus stimuli that at one time might be considered relevant will at another time be considered noise, depending on what one is doing at the moment. In recent years there has been a great deal of interest in the effects of noise on human behavior, and concepts such as “noise pollution�?have arisen, together with movements to reduce noise.

　　Exposure to loud noises can definitely produce a partial or complete loss of hearing, depending on the intensity, duration, and frequency composition of the noise. Many jobs present noise hazards, such as working in factories and around jet aircraft, driving farm tractors, and working (or sitting) in music halls where rock bands are playing. In general, continuous exposure to sounds of over 80 decibels (a measure of the loudness of sound) can be considered dangerous. Decibel values correspond to various sounds. Sounds above about 85 decibels may, if exposure is for a sufficient period of time, produce significant hearing loss. Actual loss will depend upon the particular frequencies to which one is exposed, and whether the sound is continuous or intermittent.

　　Noise can have unexpected harmful effects on performance of certain kinds of tasks, for instance, if one is performing a watch keeping task that requires vigilance, in which he is responsible for detecting weak signals of some kind (e.g., watching a radar screen for the appearance of aircraft).

　　Communicating with other people is unfavorably affected by noise. If you have ridden in the rear of a jet transport, you may have noticed that it was difficult to carry on a conversation at first, and that, eventually, you adjusted the loudness of your speech to compensate for the effect. The problem is noise.

　　20. Noise differs from sound in that ________.

　　[A] it is sounds that interfere with the task being done

　　[B] it is a special type of loud sound

　　[C] it is usually unavoidable in big cities(A)

　　[D] it can be defined more precisely than the latter

　　21. One of the harmful effects of noise on human performance is that ________.

　　[A] it reduces one’s sensitivity

　　[B] it renders the victim helpless

　　[C] it deprives one of the enjoyment of music(A)

　　[D] it drowns out conversations at worksites

　　22. The purpose of this passage is ________.

　　[A] to define the effects of noise on human behavior

　　[B] to warn people of the danger of noise pollution

　　[C] to give advice as to how to prevent hearing loss(A)

　　[D] to tell the difference between noise and sound

　　�{�案�Q?0. [A]21. [A]22. [A]