At some time in your life you may have a strong desire to do something strange or terrible. However, chances are that you don't act on your impulse , but let it pass instead. You know that to take the action is wrong in some way and that other people will not accept your behavior. Perhaps the most interesting thing about the phenomenon of taboo behavior is how it can change over the years within the same society, how certain behavior and attitudes once considered taboo can become perfectly acceptable and natural at another point in time. Topics such as death, for example, were once considered so upsetting and unpleasant that it was a taboo to even talk about them. Now with the publication of important books such as On Death and Dying and Learning to Say Goodbye, people have become more aware of the importance of expressing feelings about death and, as a result, are more willing to talk about this taboo subject. One of the newest taboos in American society is the topic of fat. Unlike many other taboos, fat is a topic that Americans talk about constantly. It's not taboo to talk about fat; it's taboo to be fat. The "in" look is thin, not fat. In the work world, most companies prefer youthful-looking, trim executives to sell their image as well as their products to the public. The thin look is associated with youth, vigor, and success. The fat person, on the other hand, is thought of as lazy and lacking in energy, self-discipline, and self-respect. In an image-conscious  society like the U.S., thin is "in", fat is "out". It's not surprising, then, that millions of Americans have become obsessed with staying slim and "in shape". The pursuit of a youthful physical appearance is not, however, the only reason for America's fascination with diet and exercise. Recent research has shown the critical importance of diet and exercise for personal health. As in most technologically developed nations, the life-style of North Americans has changed dramatically during the course of the last century. Modern machines do all the physical labor that people were once forced to do by hand. Cars and buses transport us quickly from point to point. As a result of inactivity and disuse, people's bodies can easily become weak and vulnerable to disease. In an effort to avoid such a fate, millions of Americans are spending more of their time exercising. The main reason the passage gives for why so many Americans are exercising regularly is  _  .
their changed life-style their eagerness to stay thin and healthy their appreciation of the importance of exercise the encouragement they have received from their companies