Medical researcher: Scientists compared a large group of joggers who

I picked E because of two reasons:
1) It adds another factor i.e. severity of the injuries, that could vary and people who exercise could lessen the severity of injuries by exercising.
2) Since outside information is allowed, we could categorise jogging in certain exercises.

Can anyone clarify me on this one?

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Because (E) isn't about jogging itself. We want to weaken the claim that stretching doesn't help us during jogging. The answer says "some forms". What does this mean? Is jogging a part of this? We have no idea. You cannot assume that jogging is a part of this group because you could make the equally valid claim that it isn't. You cannot make assumptions like this to validate your answer.

Why is C wrong? If most of the jogging injuries are not caused by stretching then the author cannot make that statement about stretching not being useful.

C is actually strengthen the argument. It implies that stretching has little to do with the kind of injuries, such as falls, collisions ..
Therefore, if the flexibility warm up doesn't help, it echoes the medical researcher argument that the stretch doesn't help to reduce injuries.

Medical researcher: Scientists compared a large group of joggers who habitually stretch before jogging to an equal number of joggers who do not stretch before jogging. Both groups of joggers incurred roughly the same number of injuries. This indicates that stretching before jogging does not help to prevent injuries.

Which one of the following, if true, would most weaken the medical researcher’s argument?

(A) For both groups of joggers compared by the scientists, the rate of jogging injuries during the study was lower than the overall rate of jogging injuries. - WRONG. Numbers and rate are non-comparable.
(B) Among the joggers in the groups compared by the scientists, many of those previously injured while jogging experienced difficulty in their efforts to perform stretches. - WRONG. The passage core still remains intact as these people must have been segregated as suggested.
(C) Most jogging injuries result from falls, collisions, and other mishaps on which the flexibility resulting from stretching would have little if any effect. - WRONG. Strengthens actually.
(D) The more prone a jogger is to jogging injuries, the more likely he or she is to develop the habit of performing stretches before jogging. - CORRECT.
(E) Studies have found that, for certain forms of exercise, stretching beforehand can reduce the severity of injuries resulting from that exercise. - WRONG. For two red-text parts

Sadly, D is the answer but I don't think it has merit, especially the highlighted text. Developing habit is concerned with jogger not the study.
It would have been better had the choice given some light to how the two compared to each other at the start of the study and how it was at the end of it.
True weakener would have been the one if it had elaborated that the stretching joggers were more compared to non-stretching joggers.

Answer D.

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