Critical Reasoning

Researchers have found that when very overweight people, who tend to have relatively low metabolic rates, lose weight primarily through dieting, their metabolisms generally remain unchanged. They will thus burn significantly fewer calories at the new weight than do people whose weight is normally at that level. Such newly thin persons will, therefore, ultimately regain weight until their body size again matches their metabolic rate.

The conclusion of the argument above depends on which of the following assumptions?

(A) Relatively few very overweight people who have dieted down to a new weight tend to continue to consume substantially fewer calories than do people whose normal weight is at that level.
(B) The metabolisms of people who are usually not overweight are much more able to vary than the metabolisms of people who have been very overweight.
(C) The amount of calories that a person usually burns in a day is determined more by the amount that is consumed that day than by the current weight of the individual.
(D) Researchers have not yet determined whether the metabolic rates of formerly very overweight individuals can be accelerated by means of chemical agents.
(E) Because of the constancy of their metabolic rates, people who are at their usual weight normally have as much difficulty gaining weight as they do losing it.

OA is A

Few comments: Question is hard with the way they have constructed and took me long time to paraphrase and got to its skeletpon. It gives a good insight how a question can be turned into a complicated one. I could come down to A and D and chose the wrong answer even after I had predicted something close to the correct answer choice.

Neverthless, on a second analysisit was more puzzling. I do understand the correct answer and can feel the author making this appropriate assumption. I hope no one ever did say so in reality otherwise overweight people might have felt helpless. Anyways.

Question:

1. Isn't the answer choice D a protector of the argument that mean if negated , it destroys the argument: (simplfiied form)
a new chemical agent is found that can accelerate the metabolism rate. so "such newly thin person" won't get overweight ultimately. woww... the author of the argument is in trouble.

2. Well, to argue a bit more and to ensure I have not misunderstood, i put C also for discussion here for everyone's benefit.I see this as a weakener in its stated position so can't be a contendor even. If it iw energy consumer and not the actual weight that is decisive factor, then gaining weight depends on eating factor .

Negated statement : the amount of burnt calory is determined more by weight than by energy consumed.

that means two cases (overweight who has got new weight and someone with normal weight at that level) have different metabolism so it strengthens the argument.So wrong.

help please.