Exercise is one of the few factors with a positive role in long-term control 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 efforts. The Centers for Disease Control, for example, found that fewer than one-fourth of overweight adults who were trying to lose weight said they were combining exercise with their diet.
In rejecting exercise, some people may be discouraged too much by caloric-expenditure（热量消耗）charts; for example, one would have to quickly walk hard three miles just to work off the 275 calories in one delicious Danish pastry(小甜饼). Even exercise professionals accept 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 hate "light" beer and low-calorie bread.

B．They usually ignore the effect of exercise on losing weight.

C．Most of them were combining exercise with their diet.

D．They know the actors that play a positive role in keeping down body weight.

"Even exercise professionals accept half a point here" means "They          ".

A．agree that the calories in a small piece of pastry can be easy to work off by exercise

B．partially believe diet plays a supporting role in weight reduction

C．are not fully convinced that dieting can help maintain one's new weight

D．are not sufficiently informed of the positive role of exercise in losing weight

What was confirmed by the Boston University Medical Center's study?

A．Controlling one's calorie intake is more important than doing exercise.

B．Even occasional exercise can help reduce weight.

C．Weight reduction is impossible without exercise.

D．One could lose ten pounds in a year's time if there's no increase in food intake.

What is the author's purpose in writing this article?

A．To stress the importance of maintaining proper weight.

B．To support the statement made by York Onnen.

C．To show the most effective way to lose weight.

D．To introduce the study of the Boston University Medical Center.