题目内容：
Questionsare based on the following passage:In the United States, 30 percent of the adult population has a "weight problem". To many people, the cause is obvious:they eat too much. But scientific evidence does little to support this idea. Going back to the America of the 1910s, we find that people were thinner than today, yet they ate more food. In those days people worked harder physically, walked more, used machines much less and didn't watch television.
Several modern studies, moreover, have shown that fatter people do not eat more on the av-erage than thinner people. In fact, some investigations, such as the 1979 study of 3, 545 London office workers, report that, on balance, fatter people eat less than slimmer people. Studies show that slim people are more active than fat people. A study by a research group at Stanford University School of Medicine found the following interesting facts:
The more the men ran, the more body fat they lost.
The more they ran, the greater amount of food they ate.
(76) Thus, those who ran the most ate the most, yet lost the greatest amount of body fat.
The physical problem that many adult Americans have is that__________. A．they are too slim
B．they work too hard
C．they are too fat
D．they lose too much body fat
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