As people continue to grow and age, our body systems continue to change. At a certain point in your life your body systems will begin to weaken. Your strength may become weaker. 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 change that leads us toward the end of our lives. Many factors decide your health. A good diet plays an important role. The amount and type of exercise you get is another factor. Your living condition is yet another. But scientists studying 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 old age comes. Many consider the later part of life to be the best time for living. Physical activity may become less, but often you get better understanding of yourself and the world. What we consider old age now may only be middle age some day soon. Who knows, with so many advances in medical science happening so quickly, life span may one day be measured in centuries, rather than in years. According to the passage, "span" means ______.

A. longevity
B. a length of time
C. a long period of time
D. a long distance from one place to another