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Almost everybody wants to live as long as possible. And given the enormous strides made in medicine and the health sciences duri

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问题    Almost everybody wants to live as long as possible. And given the enormous strides made in medicine and the health sciences during the past 150 years, people could be forgiven for hoping that someday human beings will live, if not quite forever, at least far longer than at present. Since the mid 19th century, average life expectancy at birth has nearly doubled: from 40 years to 75. Today many people live past 100, and the oldest individuals have reached either 115 or 120, depending on whom you believe.
   So it comes as something of a surprise to be told by the experts that human beings have taken life about as far as it can go. That is the serious conclusion of a report in Science magazine last week by S. Jay Olshansky and Christine Cassel. Unless an unexpected breakthrough in basic science that would prevent the aging process, the era of rapid increases in human life span has come to an end, at least in developed countries. Even if science could cure heart disease and cancer, which account for nearly 50 percent of all deaths in the U. S. , it is unlikely that the average life expectancy at birth would increase much beyond 85.
   What makes the report so compelling is that it is based on simple mathematics. In the past, the upper limits of life have been guessed from actuarial tables by estimating how death rates would change if, say, the incidence of heart disease were halved. "We reversed the question," says Olshan sky. Taking an" engineering approach" his team members asked themselves how many death rates would have to be reduced in order to increase average life expectancy to 120 years. What they discovered, after running the numbers through a computer, was that big hits in current death rates in the U. S. would give only small lifts to life expectancy. For example, if some miracle of medicine can guarantee no one died before reaching age 50 (thus eliminating 12 percent of all deaths), the increase in average life expectancy would be only 3.5 years.
   There seems to be a kind of built-in biological limit programmed into the cells of the human body. In laboratory experiments, human cells divide only about 50 times before they begin to fall apart like old cars. This planned loss of use on nature’s part makes a certain amount of evolutionary sense. Survival of the fittest, after all, rewards only those who reproduce, not necessarily those who reach old age, Once reproduction is over, human bodies may as well be throwaway goods, biologically speaking.
What Olshansky and his team member discovered shows that
选项 A、it is possible for the average life span to increase to 120 years, if death rates are reduced.
B、the average human life span will increase a little, if the death rates are reduced.
C、the upper limits of life bears no relation with the change of death rates,
D、the average human life span will increase a lot, if no one died before reaching age 50.
答案B
解析 根据文章第三段第四句可知他们的研究：如果要把人的平均寿命提高到120岁，应该把死亡率减少到多少。他们的研究发现即使是给死亡率带来巨大负面影响的疾病减少了也只能带来人寿命的小幅度延长。A与文意不符，降低死亡率并不能使人类寿命延长到120岁成为可能。C与文意不符，这不是Olshansky他们的研究发现。D与文意不符，本段最后一句说如果医药方面的有什么奇迹能使得没有人死于50岁之前，在这种情况下人的平均寿命才能延长三年半，也就是说不会延长很多。
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