It is generally believed that people need to engage in regular and strenuous physical exercise in order to maintain their physical health. Actually, however, low-impact walking —that is, walking at a normal pace, which elevates the heart rate and releases many of the hormones released in more strenuous exercise—is enough. This is evidenced by the strong correlation between the amount of time spent walking and measures of general physical health demonstrated by many studies.
Which of the following, if true, most seriously weakens the force of the evidence cited?
A. Most doctors advise that people begin more strenuous and intense physical exercises if they wish to improve their conditioning.
B. People who already perform strenuous physical exercises also tend to walk more than people who do not.
C. People who are fond of strenuous physical exercise also tend to be fond of walking.
D. The studies mentioned did not gather their own data but summarized data collected from other studies.
E. The physical activities described as "strenuous" varied wildly between studies.
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It says walking is enough to simulate the effects of strenuous workout. And the study co-relates '' time spent walking and measures of general physical health''.

As per B People who already perform strenuous physical exercises also tend to walk more than people who do not. . So they should have superior health compared to those who just walks because these guys walk more than anyone else. But the argument says walking is enough to have comparable effect.