We need to weaken the force of given evidence which is --- by the strong correlation between the amount of time spent walking and measures of general physical health demonstrated by many studies.
Pre-think:may be time has a lot to do with this. Also metabolism works in this case. So you need some exercise or a lot of walking or a bit of both. So focus need to be that low impact walking is not enough.
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. --- may be true but this is not doing anything to conclusion
B. People who already perform strenuous physical exercises also tend to walk more than people who do not.--- So this one is a case of bit of both. Walking might be an add-on but actually the exercise is enough for good health, though we cant say for certain but impact of walking is skeptical. but this case is saying that such people have better physical health.
C. People who are fond of strenuous physical exercise also tend to be fond of walking. --- just being the fond of something and not doing it actually has a lot of difference.
D. The studies mentioned did not gather their own data but summarized data collected from other studies. --- This is out of scope.
E. The physical activities described as "strenuous" varied wildly between studies. ---- Still these studies are benchmark. and WRT premise, they are well proven.
B is best from the lot.
_________________
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Simple strategy:
“Once you’ve eliminated the impossible, whatever remains, however improbable, must be the truth.”
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61% (01:41) correct 
