1 . Throughout history man has had to accept the that all living things must die. But people now live longer than they     ____. Yet, all living things still show the________of aging, which will eventually ______ death.

Aging is not a disease, but as a person passes maturity (成熟期), the cells of the body and the ____ they form do not function as well as they ______ in childhood and teenage years. The body provides less ______ against disease and is more _______to have accident.

A number of related causes may________aging. Some cells of the body have a fairly long life, but they are not     _____when they die. As a person ages,________of brain cells and muscle cells decreases. ______ body cells die and are replaced by new cells. In an aging person the_______cells may not be as workable or as capable ______ growth as those of a young person.

Another ______ in aging may be changes within the cells ______ . Some of the protein chemicals in cells are known ______ with age and become less elastic (有弹性的). This is why the skin of old people wrinkles (皱纹) and     ____. This is also the reason why old people________in height. There may be other more important chemical changes in the cells. Some complex cell chemicals, such as DNA and RNA, store and________information that the cells need. Aging may affect this______and change the information-carrying molecules so that they do not transmit the information as well.