If athletes want better performances, they should train at high altitudes. At higher altitudes, the body has more red blood cells per unit volume of blood than at sea level. The red blood cells transport oxygen, which will improve performance if available in greater amounts. The blood of an athlete who trains at high attitudes will transport more oxygen per unit volume of blood, improving the athlete's performance.

Which of the following, if true, would be most damaging to the argument above, provided that the athlete's heart rate is the same at high and low altitudes?

(A) Scientists have found that an athlete's heart requires a period of time to adjust to working at high altitudes.
(B) Scientists have found that the body's total volume of blood declines by as much as 25 percent at high altitudes.
(C) Middle-distance runners who train at high altitudes sometimes lose races to middledistance runners who train at sea level.
(D) The performances of athletes in competitions at all altitudes have improved markedly during the past twenty years,
(E) At altitudes above 5,500 feet, middle-distance runners often better their sea-level running times by several seconds.