In a study, one group of volunteers was fed a high-protein, low-carbohydrate diet; another group was fed a low-protein, high-carbohydrate diet. Both diets contained the same number of calories, and each volunteer's diet prior to the experiment had contained moderate levels of proteins and carbohydrates. After ten days, those on the low-carbohydrate diet had lost more weight than those on the high-carbohydrate diet. Thus, the most effective way to lose body fat is to eat much protein and shun carbohydrates.

Question: Which one of the following, if true, most weakens the argument above?
Options:
A. A low-protein, high-carbohydrate diet causes the human body to retain water, the added weight of which largely compensates for the weight of any body fat lost, whereas a high-protein, low-carbohydrate diet does not.
B. Many people who consume large quantities of protein nevertheless gain significant amounts of body fat.
C. A high-protein, low-carbohydrate diet will often enable the human body to convert some body fat into muscle, without causing any significant overall weight loss.
D. In the experiment, the volunteers on the high-carbohydrate diet engaged in regular exercise of a kind known to produce weight loss, and those on the low-carbohydrate diet did not.
E. Many of the volunteers who had been on the low-carbohydrate diet eventually regained much of the weight they had lost on the diet after returning to their normal diets.
Answer: "A"