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  3. It is an everyday observation that animals are born, grow, and mature then begin to lose some of their capabilities, and finally

It is an everyday observation that animals are born, grow, and mature then begin to lose some of their capabilities, and finally

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问题     It is an everyday observation that animals are born, grow, and mature then begin to lose some of their capabilities, and finally die. This loss of capabilities is progressive, irreversible, and common for all members of a species and is called aging. In humans it shows in many ways, some very visible: decline in height, shrinkage of muscle, thinning and graying of hair, and wrinkling of skin. Internally, and even more significantly, there is the progressive loss of cells in the brain, kidneys, and other vital organs. This cell loss has resulted in stoppage of a vital process. Whatever the reason, important tissues ranging from the muscles to the brain shrink and become less competent with age. Many of these changes are reflected in functional declines. Not only do nerve cells at levels from the brain to the spinal cord(脊髓)diminish, but also those that remain conduct impulses at a slower rate so that the reaction time of older animal is slowed. Memory often shows a decline.

    Another widespread decline is in the loss of cells involved in the hearing process. The loss is most marked for high pitches and may require the assistance of a battery-operated hearing aid. More subtle are such declines as those in the processes involved with being immune to disease, which result in a lessened ability of the older organism to cope with infection. Indeed, pneumonia(肺炎)is one of the most common causes of death among the elderly population of most nations.
    Mixed with true aging processes are disease processes that may be so common as to be mistaken for aging. For instance, the buildup of deposits of fatty materials in arteries tends to be progressive with aging; everything else being equal, the narrowing of arteries results in such serious illnesses as stroke or heart attack, occurring with increasing frequency as an individual ages.
    It is now recognized that atherosclerosis(动脉硬化症)is the result of many factors, not only genetic but also environmental high blood pressure, high saturated-fat diets, and smoking, the effects of which become more obvious with the passage of time. It is therefore an age-related, but not a universal aging process and can thus usually be controlled.
Pneumonia, which is one of the most common causes of death among the elderly population, is closely related to the decline in the______system.
选项
答案immune
解析 根据关键词Pneumonia，找到原文第二段最后一句话，Pneumonia是对于前面内容的举例，免疫功能的衰退引起老年人身体抗感染能力下降，才致使肺炎成为导致老年人死亡的最普遍原因之一，所以答案为immune。
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