Like distance runners on a measured course，all of us will move through time in a roughly predictable pattern．
In the first stage of our lives，we develop and grow, reaching toward the top of physical vitality(活力)．
After we grow up，however, the body begins a process of gradually wearing out．
A new awareness of physical fitness may help lengthen our years of health and vitality，yet nothing we do will work to stop the unavoidable force of aging．
Most of the changes of aging take place deep inside the body．The lungs bee less able to take in oxygen．Powerful muscles gradually lose their strength．The heart loses power and pumps less blood．Bones grow easier to break．
Finally, we meet a stress，a stress that is greater than our physical resistance．Often，it is only a minor accident or chance infection(a disease caused by virus)，but this time，it brings life to an end．
In 1932，a classic experiment nearly doubled the lifetime of rats，simply by cutting back the calories in their diet The reason for the effect was then unknown．
Today, at the University of California at Berkley, Dr．Paul Seagle has also greatly lengthened the normal lifetime of rats．The result was achieved through a special protein limited diet，which had a great effect on the chemistry of the brain．Seagle showed that within the brain，specific chemicals control many of the signals that influence aging．By changing that chemical balance，the clock of aging can be reset．
For the first time, the mystery of why we age is being seriously challenged．Scientists in many fields are now making striking and far-reaching discoveries．
An average lifetime lasts 75 years，yet in each of us lies a potential for a longer life．If we could keep the vitality and resistance to disease that we have at age twenty, we would live for 800 years．
【小题1】What does the underlined word “it” refer to?A．Physical vitality．B．Stress．C．Aging．D．Physical resistance．【小题2】Which of the following is not a change of aging?A．People gradually lose their muscle strength．B．Bones tend to break more easily．C．The heart loses power and pumps less blood．D．The lungs bee unable to take in oxygen．【小题3】The author believes the following except that ．A．human's life pattern is predictableB．physical fitness can't stop the force of agingC．human's lifetime will last longer than 75 yearsD．all of the changes of aging take place deep inside the body【小题4】How did Dr．Paul Seagle lengthen the lifetime of rats in his experiment?A．By keeping their physical fitness．B．By cutting the calories in their diet．C．By resetting their clock of aging．D．By limiting the protein in their diet．【小题5】What is probably the best title for the article?A．Two Great ExperimentsB．The Mystery of Human LifeC．The Breakthrough in the Study of AgingD．Ways to lengthen Human LifeB
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(答案→)B
解析：
【小题1】B 推理题。根据本句we meet a stress，a stress that is greater than our physical resistance．Often，it is only a minor accident or chance infection(a disease caused by virus)，but this time，it brings life to an end．说明这里的it指上的stress，它让我们的生命结束。
【小题2】D 细节题。根据文章第五段Most of the changes of aging take place deep inside the body．The lungs bee less able to take in oxygen．Powerful muscles gradually lose their strength．The heart loses power and pumps less blood．Bones grow easier to break．说明ABC都是变老的变化，D项表述错误，肺部不是不能吸入氧气，是吸入氧气变少。
【小题3】D 推理题。根据文章第五段Most of the changes of aging take place deep inside the body．The lungs bee less able to take in oxygen．Powerful muscles gradually lose their strength．The heart loses power and pumps less blood．Bones grow easier to break．这里很多变老的症状都表现在外在了，故D说法错误。
【小题4】D 细节题。根据文章倒数第四段a classic experiment nearly doubled the lifetime of rats，simply by cutting back the calories in their diet说明D正确。减少了饮食中的卡路里，与D项相符。
【小题5】C 主旨大意题。本文讲述了一个新的科学发现，是对人类变老的研究，结果发现我们可以延长我们的衰老过程，以延长我们的生命。故C正确。