Not necessary, because the author doesn’t have to assume that anyone “fully” benefited from aerobic exercise. Although the author does have to assume that three months is enough time to observe at least SOME of the benefits of aerobic exercise, this doesn’t require a belief about what length of time is required for FULL benefit.

Not necessary, because as long as the aerobics class group did not do MORE weight-training overall than the weight-training class group, they could still have done some weight-lifting.

Not necessary, because the author’s reasoning never relies on a comparison between the stress levels after three months to stress levels before the study started. The study wasn’t measuring an INCREASE or DECREASE in stress levels; it simply compared stress levels at 1 point in time between the aerobics class group and the weight-training class group.

Not necessary, because a comparison of difficulty is irrelevant. The point of the math calculation was to put people under some kind of stress-inducing activity so that we could measure the stress symptoms. Whether the calculation was more or less difficult for one group compared to the other doesn’t matter, because we can still measure someone’s stress after the calculation regardless of difficulty.

Necessary, because the author must believe that a greater amount of aerobics exercise was the difference that explains the differing results on the stress measurements. If (E) were not true — if the aerobics class group actually got EQUAL or LESS aerobic exercise overall than the other group — then we have no reason to think aerobic exercise played a role in the differing stress levels.