[单项选择]

Exercise is one of the few factors with a positive role in long-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. they think it is physically exhausting
B. they find it hard to exercise while on a diet
C. they don't think it possible to walk 3 miles every day
D. they find consulting caloric-expenditure charts troublesome