Cyclists in the Tour de France are extremely physically fit: all of the winners of this race have had abnormal physiological constitutions. Typical of the abnormal physiology of these athletes are exceptional lung capacity and exceptionally powerful hearts. Tests conducted on last year’s winner did not reveal an exceptionally powerful heart.
That cyclist must, therefore, have exceptional lung capacity.
The reasoning in the argument is most vulnerable to criticism on the grounds that it overlooks the possibility that
(A) having exceptional lung capacity and an exceptionally powerful heart is an advantage in cycling
(B) some winners of the Tour de France have neither exceptional lung capacity nor exceptionally powerful hearts
(C) cyclists with normal lung capacity rarely have exceptionally powerful hearts
(D) the exceptional lung capacity and exceptionally powerful hearts of Tour de France winners are due to training
(E) the notions of exceptional lung capacity and exceptional heart function are relative to the physiology of most cyclists
The last sentence of the passage is the conclusion and we need to find something that weakens it. So, it should either say something else other than lung capacity helped last year's winner win the race or that both exceptional powerful heart and exceptional lung capacity is not a necessity.
The word 'that' in the last sentence is specific to the cyclist. B makes a generic statement about cyclists that must include last year's winner. However, this generic statement does create a doubt selecting it.
On the other hand, E goes in either direction - strengthens or weakens - depending on the further assumption one makes.