题目内容：
根据以下材料，回答题Exercise is one of the few factors with a positive role in tong-term maintenance of body weight.
Unfortunately, that message has not gotten through to the average American, who would rather try switching to "light" beer and low-calorie bread than increase physical exertion. The Centers for Disease Control, for example, found that fewer than one-fourth of overweight adults who were trying to shed pounds said they were combining exercise with their diet.
In rejecting exercise, some people may be discouraged too much by caloric-expenditure charts:for example, one would have to briskly walk three miles just to work off the 275 calories in one delicious Danish pastry(小甜饼). Even exercise professionals concede half a point here. "Exercise by itself is a very tough way to lose weight," says York Onnen, program director of the President's Council on Physical Fitness and Sports.
Still, exercise's supporting role in weight reduction is vital. A study at the Boston University Medical Center of overweight police officers and other public employees confirmed that those who dieted without exercise regained almost all their old weight, while those who worked exercise into their daily routine maintained their new weight.
If you have been sedentary (极少活动的 )and decide to start walking one mile a day, the added exercise could burn an extra 100 calories daily. In a year's time, assuming no increase in food intake,you could lose ten pounds. By increasing the distance of your walks gradually and making other dietary adjustments, you may lose even more weight.
What is said about the average American in the passage? A．They tend to exaggerate the healthful effect of "light" beer
B．They usually ignore the effect of exercise on losing weight
C．They prefer "light" beer and low-calorie bread to other drinks and food
D．They know the factors that play a positive role in keeping down body weight
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