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 skin wrinkles, 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 26_______ which would determine how long the cells can remain 27_______ Depending on the type of animal and its environment, animals age at different rates and live 28_______ different lengths of time. An animal in a good zoo—well 29_______ and protected from predators（捕食者）一often lives longer than the same type in 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 is genetics （遗传）.In some families, it seems that many 33_______ have long lives. Genetics may also determine whether people 34_______ certain diseases. Another factor is lifestyle. People who keep their minds 35_______ and often communicate with friends will feel younger and may live longer. People who keep a normal weight, exercise, and do not smoke may also age more slowly.
21.
A.moreover B.besides C.therefore D.however正确答案D
