单项选择题

Throughout history man has had to accept the fact that all living things must die. But people now live longer than they(1). Yet, all living things still show the(2)of aging, which will eventually(3)death. Aging is not a disease, （but）as a person passes maturity, the cells of the body and the(4) they form do not function as well as they(5) in childhood and teenage years. The body provides less(6)against disease and is more(7)to have accident. A number of related causes may(8)aging. Some cells of the body have a （fairly） long life, but they are not(9)when they die. As a person ages,(10)of brain cells and muscle cells decreases.(11)body cells die and are （replaced） by new cells. In an aging person the(12)cells may not be as workable or as capable(13)growth as those of a young person. Another(14)in aging may be changes within the cells(15). Some of the protein chemicals in cells are known(16)with age and become less elastic. This is why the skin of old people wrinkles and(17). This is also the reason why old people(18)in height. There may be other more important chemical changes in the cells. Some complex cell chemicals, such as DNA and RNA, store and(19)information that the cells need.Aging may affect this(20)and change the information carrying molecules so that they do not transmit the information as well.

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