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Throughout history man has to accept the fact that all living things must die. Although people now live longer than they used to, yet all living things still show the ___(46)___ of aging, which will eventually result in death. Aging is not a ___(47)___, but as a person passes maturity, the cells of the body and the organs they ___(48)___ do not work as well as they did in childhood and youth. The body provides less ___(49)___ against disease and is more prone (倾向于) to ___(50)___ A number of related causes may ___(51)___ aging. Some cells of the body have a fairly long life, but they are not ___(52)___ new as well as they did in childhood and youth. The body provides less ___(49)___ against disease and is more prone (倾向于) to ___(50)___ Other body cells die and are replaced by new cells. In an aging person the new cells may not be as active or as ___(54)___ of growth as those of young people. Another factor in ___(55)___ may be changes within the cells ___(56)___ Some of the protein chemicals in cells are known to change with age and become ___(57)___ elastic. This is why the skin of old people wrinkles and hangs ___(58)___ This is also the reason why old people shrink in ___(59)___ There may be other more important chemical changes in the cells. Some complex cell chemicals, such as DNA and RNA, store and ___(60)___ information that the cells need. Aging may affect this process and change the information-carrying molecules (分子) so that they do not pass on the information as well.