As children move towards adulthood (成年), they become taller, stronger, and more independent.At some point in adulthood,?21?, a slow decline begins. Their hair often?22?gray, their skinwrinkles, and their muscles begin to?23?Their short-term memory may suffer, and they often?24?part of their vision or hearing.?
Scientists are not?25?sure what causes the effects of aging. The body might have a time?26which would determine how long the cells can remain?27?Depending on the type of animal and itsenvironment, animals age at different rates and live?28?different lengths of time. An animal in agood zoo--well?29?and protected from predators (捕食者) --often lives longer than the same typein the wild.?30?, people who live in rich countries generally live longer than?31?in poor countries.?
Several other factors also?32?how long people live and the quality of their lives. One factor isgenetics (遗传). In some families, it seems that many?33?have long lives. Genetics may alsodetermine whether people?34?certain diseases. Another factor is lifestyle. People who keep theirminds?35?and often communicate with friends will feel younger and may live longer. People whokeep a normal weight, exercise, and do not smoke may also age more slowly.?
第（21）题选
A.moreover
B.besides
C.therefore
D.however