Throughout history man has had to accept the fact that all living things must die, but people now live longer than they 21 . Yet, all living things still show the __22 of aging, which will eventually 23 death.
Aging is not a disease, but as a person passes maturity, the cells of the body and the organs they form do not function as well as they did in childhood. The body provides less 24 against diseases and is more inclined 25 accident.
A number of related causes may contribute to aging. Some cells of the body have a fairly long life, but they are not 26 when they die. In an aging person the new cells may not be as capable 27 growth as those of a young person.
Another 28 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 flexible. This is why the skin of old people wrinkles. This is also the reason old people 29 in height. There may be other more important chemicals changes in the cells. Some complex cell chemicals, such as DNA and RNA, store and 30 information that the cells need. Aging may affect this process and change the information-carrying molecules so that they do not transmit the information as well.

A.factor

B.effect

C.reason

D.element