根据下面资料，回答21-35题
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（26）题选

A label

B lack

C link

D limit