A recent study of people who had successfully lost weight and implemented regular exercise routines in their twenties found that, by the age of forty, most had gained back the weight, stopped exercising regularly, or both. Surprisingly, among the study's subjects who had hired personal trainers to help them lose weight and exercise in their twenties, an even higher percentage had regained the weight or stopped exercising than among subjects who had not hired such trainers. The researchers concluded not only that remaining on an effective weight management and exercise program is difficult, but also that personal trainers are largely ineffective in helping people to remain on an effective weight management and exercise program.

Which of the following, if true, most weakens the argument in the passage above?

(A) Subjects in the study who had hired personal trainers had, on average, more free time to exercise than did those who had not hired such trainers.
(B) Some people hire personal trainers for sport-specific training or to rehabilitate injuries, rather than to lose weight or implement a regular exercise program.
(C) The average person's metabolism slows significantly between the ages of thirty and forty, making it more difficult for people aged forty or older to avoid weight gain.
(D) Many of the personal trainers hired by the people in the study were also dietitians who helped their clients design meal plans.
(E) Most people who hire personal trainers do so, at least in part, because they lack sufficient motivation to remain on a diet or exercise regimen by themselves.