The amount and type of exercise you get is another factor．Your living environment and the

amount of stress you are under is yet another．But scientists studying  senescence（哀老）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.

 21．When people become aging，they will lose some of their ability to bounce back from

    disease and injury，“bounce back”  here means____．

    A．to imProve in health after one’s disease and injury

    B．to recover from disease and injury

    C．to jump after recovering

    D．to run fast

22．In  order to  live  longer，_____.

    A．we have to try to be on a diet

    B．We Should keep in high spirits

    C．we should try to do more exercise

    D．we should postpone the process of aging

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