A number of related causes may__18__to aging. Some cells of the body have a fairly long life, but they are not__19__when they die. As a person ages，__20__of brain cells and muscle cells decreases.__21__body cells die and are replaced by new cells. In an aging person the__22__cells may not be as workable or as capable__23__growth as those of a young person.

　　Another__24__in aging may be changes within the cells__25__. Some of the protein chemicals in cells are known__26__with age and become less elastic (有弹性). This is why the skin of old people wrinkles and__27__. This is also the reason why old people__28__in height. There may be other more important chemical changes in the cells. Some complex cell chemicals, such as DNA and RNA, store and__29__information that the cells need. Aging may affect this__30__and change the information­carrying molecules so that they do not transmit the information as well.

　　11．A.would

　　B．be used to

　　C．used to

　　D．used

　　12．A.function

　　B．effect

　　C．affect

　　D．sign

　　13．A.lead in

　　B．give in

　　C．run into

　　D．result in

　　14．A.hands

　　B．feet

　　C．heart

　　D．organs

　　15．A.do

　　B．has done

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