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A number of related causes s may 48 aging.Some cells of the body have a fairly long life,but they can't be 49 when they die.As a person ages,the 50 of brain cells and muscle cells decreases.Other body cells 51 and are replaced by new cells.In an aging person the 52 cells may not be as workable or as capable of 53 as those of a young person. Another 54 in aging may be changes within the cells themselves.Some of the protein chemicals in cells are known to 55 with age and become less elastic(有弾性的).This is why the skin of old people wrinkles (and becomes 56 This is also the reason why old people e 57 in height.There may be other more 58 chemical changes in the cells.Some complex cell chemicals,such as DNA and RNA,store and 59 information that