As children move towards adulthood(成年),they become taller,stronger,and more independent At some point in adulthood, 21 ,a slow decline begins.Their hair often 22 g hr skin wrinkles,and their muscles begin to 23 Their short-term memory may suffer,and they often 24 part of their vision or hearing.
Scientists are not 25 sure what causes the effects of aging.The body might have a time26 which would determine how long the cells can remain 27_.Depending on the type of animal and its environment,animals age at different rates and live 28 different lengths of time.An animal in a good zoo-well 29_and protected from predators(捕食者)—often lives longer than the same type in the wild. 30_,people who live in rich countries generally live longer than 31 in poor countries.
genetics(遗传) .In some families,it seems that many 33 have long lives. Genetics may also
determine whether people 34 P w h
minds 35 and often communicate wih friends will feel younger and may live longer. People who keep a normal weight,exercise,and do not smoke may also age more slowly. 21填
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