​Directions: For each blank in the following passage, there are four choices marked A, B, C and D. Choose the one that is most suitable and mark your answer by blackening the corresponding letter on the Answer Sheet.

     As children move towards adulthood（成年）, they become taller, stronger, and more independent. At some point in adulthood, , a slow decline begins. Their hair oftengray, their skin wrinkles, and their muscles begin to. Their short-term memory may suffer, and they oftenpart of their vision or hearing.
     Scientists are notsure what causes the effects of aging. The body might have a timewhich would determine how long the cells can remain. Depending on the type of animal and its environment, animals age at different rates and livedifferent lengths of time. An animal in a good zoo-welland protected from predators（捕食者）—often lives longer than the same type in the wild. , people who live in rich countries generally live longer thanin poor countries.
     Several other factors alsohow long people live and the quality of their lives. One factor is genetics（遗传）. In some families, it seems that manyhave long lives. Genetics may also determine whether peoplecertain diseases. Another factor is lifestyle. People who keep their mindsand 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.