No one really knows how and why people change as they get older. Also, no theory sufficiently explains all the changes of the aging process. Aging is a complex and varied process that varies in how it affects different people and organs. In fact, even in one person , different organ systems "age " at a different rate. At a certain point in our lives our body systems will begin to weaken . It may become more difficult for us to see and hear. The slow change of aging causes our bodies to lose some of their ability to _ from disease and injury. In order to live longer , we have always tried to slow or stop this change that leads us toward the end of our lives. Many factors decide our health . A good diet plays an important role. The amount and the type of exercise we get are another two factors. Our living condition is yet another. But scientists studying the aging problem want to know: Why do people grow old? They hope that by studying the aging medical science they may be able to make the length of life longer. There is nothing to be afraid of as the old age comes. Many consider the later part of life to be the best time for living. Physical activity may become less, but often we get better understanding of the world and ourselves. What we consider the old age now may only be middle-aged some day soon. Who knows ,with so many advances in medical science happening so quickly, life length may one day be measured in centuries ,rather than in years! In order to make people live longer, scientists   _  according to the text.
teach people how to eat properly discover how important the exercise is to one's health study the ways to slow or stop the process of aging invent new medicine to slow down the process of aging