Throughout history man has had to accept the fact that all living things must die, but people now live longer than they 【C1】______. Yet, all living things still show the【C2】______of aging, which will eventually【C3】______death. Aging is not a disease, but as a person passes maturity, the cells of the body and the【C4】______they form. do not function as well as they【C5】______in childhood and adolescence (青春期), The body provides less【C6】______against disease and is more inclined【C7】______accident. A number of related causes may【C8】______aging. Some cells of the body have a fairly long life, but they are not【C9】______when they die. As a person ages,【C10】______of brain cells and muscle cells decreases.【C11】______body cells die and are replaced by new cells. In an aging person the【C12】______cells may not be as viable (能生存的)or as capable【C13】______growth as those of a young person. Another【C14】______in aging may be changes within the cells【C15】______. Some of the protein chemicals in cells are known【C16】______with age and become less elastic. This is why the skin of the old people wrinkles and【C17】______. This is also the reason old people【C18】______in height. There may be other more important chemical changes in the cells. Some complex cell chemicals, such as DNA and RNA, store and【C19】______information that the cells need. Aging may affect this【C20】______and change the information-carrying molecules so that they do not transmit the information as well. 【C1】