Aging

Throughout historyman has had to accept the fact that all living things must die,but people nowlive longer than they used to.Yet all living things still show the________(51)

of aging,which willeventually________(52)death.

Aging is not adisease,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 bodyprovides less________(54)against disease and is more likely________(55)haveaccident.

A number of relatedcauses may________(56)aging. Some cells of the body have a fairly long life,butthey are not reborn when they die.As a person ages,________(57)of

brain cells andmuscle cells decreases.The other cells are replaced by new cells.In an agingperson the________(58) cells and muscle cells may not be as viable（能生存的）

or ascapable________ ( 59 ) growth as those of a young person.

Another________(60)inaging may be changes within the cells themselves.Some of the protein chemicalsin cells are known________(61)with age and became less

elastic.This is whythe skin of old people wrinkles and________(62).This is also the reason oldpeople________(63)in height. There may not be other more important

chemical changes inthe cells.Some complex cell chemicals, such as DNA and RNA,storeand________(64)information that the cells need.Aging may affect this________

(65)and change theinformation-carry molecules so that they do not transmit the information aswell._________(63)

A.shorten

B.shrink

C.lengthen

D.decrease