People who live unusually long tend to have been lean young

People who live unusually long tend to have been lean young adults who went on to gain approximately one pound every year, so lean young adults can improve their chances of living a long life by gaining about a pound every year.
A flaw in the argument above is that it

(A) gives reasons for the truth of its conclusion that presuppose the truth of that conclusion
(B) proceeds as though a condition that by itself is enough to guarantee a certain result must always be present for that result to be achieved
(C) assumes without proof that two phenomena that occur together share an underlying cause
(D) concludes that one phenomenon is the cause of another when at most what has been established is an association between them
(E) fails to recognize that a tendency widely shared by a subgroup within a given population will not necessarily be widely shared by that population as a whole