题目内容：
共用题干Aging
Throughout history man has had to accept the fact that all living things must die,but people now live longer than they used to.Yet all living things still show the________(51)of aging,which will eventually________(52)death.
Aging is not a disease,but as a person passes maturity,the cells of the body and the________ (53) do not function as well as they did in childhood and adolescence（青春期）.The body provides less________(54)against disease and is more likely________(55)have accident.
A number of related causes may________(56)aging. Some cells of the body have a fairly long life,but they are not reborn when they die.As a person ages,________(57)of brain cells and muscle cells decreases.The other cells are replaced by new cells.In an aging person the________(58) cells and muscle cells may not be as viable（能生存的）or as capable________ ( 59 ) growth as those of a young person.
Another________(60)in aging may be changes within the cells themselves.Some of the protein chemicals in cells are known________(61)with age and became less elastic.This is why the skin of old people wrinkles and________(62).This is also the reason old people________(63)in height. There may not be other more important chemical changes in the cells.Some complex cell chemicals, such as DNA and RNA,store and________(64)information that the cells need.Aging may affect this________(65)and change the information-carry molecules so that they do not transmit the information as well.
_________(53)
A:arm
B:feet
C:heart
D:organs
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