单项选择题

As people continue to grow and age, our body systems continue to change. At a certain point in your life, your body system began to weaken. Your joint may become stiff. It may become more difficult for you to see and hear. The slow change of aging causes our bodies to lose some of their ability to bounce back from disease and injury. In order to live longer, we have always tried to slow or stop this process that leads us toward the end of our lives. Many factors contribute to your health. A well-balanced diet plays an important role. The amount and type of exercises you get is another factor. Your living environment and the amount of stress you are under is yet another. But scientists studying senescence want to know: Why do people grow old They hope that by examining the aging process on a cellular level medical science may be able to extend the length of life. There is nothing to be afraid of as old age approaches. Many consider the later portions of life to be the best time for living. Physical activity may lessen, but often you gain a broader understanding of yourself and the world. What we consider old age now may only be middle-aged someday soon. Who knows with so many advances in medical science happening so quickly, life spans may one day be measured in centuries, rather than in years!

When people become aging, they will lose some of their ability to bounce back from disease and injury, “bounce back” here means （）.

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