完形填空(阅读以下材料 回答第51——65题)AgingThroughout history ma

完形填空(阅读以下材料，回答第51——65题)

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.

_________(51)

A.function

B.effect

C.fact

D.symbol