【单选题】

Throughout history man has had to accept the fact that all living things must die, but people now live longer than they (36) . Yet, all living things still show the (37) of aging, which will eventually. (38) death.

Aging is not a disease, but as a person passes maturity, the cells of the body and the (39) they form do not function as well as they (40) in childhood and adolescence (青春期). The body provides less (41) against disease and is more prone (42) accident.
A、number of related cause may (43) aging. Some cells of the body have a fairly long life, but they are not (44) when they die.As a person ages, (45) of brain cells and muscle cells decrease, (46) body cells die and are replaced by new cells. In an aging person the (47) cells may not be as viable (能生存的) or as capable (48) growth as those of a young person.
Another (49) in aging may be changes within the cells (50) . Some of the protein chemicals in cells are known (51) with age and become less elastiC、This is why the skin of old people wrinkles and (52) . This is also the reason old people (53) in height. There may be other more important chemical changes in the cells. Some complex cell chemicals, such as RNA, store and (54) information that the cell neeD、Aging may affect this (55) and change the information carrying molecules so that they do not transmit the information as well.
A、method B、procedure C、developmentD、process