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  3. Throughout history man has had to accept the fact that aU living things must die．But people now live longer than they used to．Ye

Throughout history man has had to accept the fact that aU living things must die．But people now live longer than they used to．Ye

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问题     Throughout history man has had to accept the fact that aU living things must die．But people now live longer than they used to．Yet，all living things still show the【C1】______of aging，which will eventualIy【C2】______death．
    Aging is not a disease，but as a person passes maturity(成熟期)，the cells of the body and the organs they form do not function as well as they【C3】______in childhood and teenage years．The body provides less energy against disease and is more likely to have accident．
    A number of related causes may【C4】______aging．Some ceils of the body have a fairly long life，but they are not replaced when they die．As a person ages，【C5】______of brain cells and muscle cells decreases．Other body cells die and are replaced by new cells．In an aging person the new cells may not be as workable or as capable of growth as those of a young person．
    Another【C6】______in aging may be changes within the cells themselves．Some of the protein chemicals in cells are known to change with age and become less elastic(有弹性的)．This is why the skin of old people wrinkles(皱纹)and【C7】______．This is also the reason why old people【C8】______in height．There may be other more important chemical changes in the cells．Some complex cell chemicals，such as DNA and RNA，store and【C9】______information that the cells need．Aging may affect this【C10】______and change the information—carrying molecules so that they do not transmit the information as well．
【C3】
选项 A、do
B、has done
C、did
D、had done
答案C
解析 此句中的did是用来代替前面的动词function，以避免重复。因为这里有一个表示过去的时间状语in childhood and teenage years，所以只能用一般过去时。
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