A选项exclusively of protein是指含有蛋白质还是不含蛋白质亚？

看了gmat阅读精解的解释，说a是只含蛋白质，根据文章最后一句话所以是最不可能的。可是我查这个词组的意思，怎么觉得a的意思是除了蛋白质之外亚

It was once believed that the brain was independent

of metabolic processes occurring elsewhere in the body.

In recent studies, however, we have discovered that the

production and release in brain neurons of the neuro-

(5) transmitter serotonin (neurotransmitters are compounds

that neurons use to transmit signals to other cells)

depend directly on the food that the body processes.

Our first studies sought to determine whether the

increase in serotonin observed in rats given a large injec-

(10)tion of the amino acid tryptophan might also occur after

rats ate meals that change tryptophan levels in the

blood. We found that, immediately after the rats began

to eat, parallel elevations occurred in blood tryptophan,

brain tryptophan, and brain serotonin levels. These find-

(15) ings suggested that the production and release of sero-

tonin in brain neurons were normally coupled with

blood-tryptophan increases. In later studies we found

that injecting insulin into a rat’s bloodstream also caused

parallel elevations in blood and brain tryptophan levels

(20) and in serotonin levels. We then decided to see whether

the secretion of the animal’s own insulin similarly affected

serotonin production. We gave the rats a carbohydrate-

containing meal that we knew would elicit insulin secre-

tion. As we had hypothesized, the blood tryptophan

(25) level and the concentrations of tryptophan

serotonin in the brain increased after the meal.

Surprisingly, however, when we added a large

amount of protein to the meal, brain tryptophan and

serotonin levels fell. Since protein contains tryptophan,

(30) why should it depress brain tryptophan levels? The

answer lies in the mechanism that provides blood tryp-

tophan to the brain cells. This same mechanism also

provides the brain cells with other amino acids found in

protein, such as tyrosine and leucine. The consumption

(35) of protein increases blood concentration of the other

amino acids much more, proportionately, than it does

that of tryptophan. The more protein in the meal, the

lower is the ratio of the resulting blood-tryptophan

concentration to the concentration of competing amino

(40) acids, and the more slowly is tryptophan provided to

the brain. Thus the more protein in a meal, the less

serotonin subsequently produced and released.

8. It can be inferred from the passage that which of the

following would be LEAST likely to be a potential

source of aid to a patient who was not adequately

producing and releasing serotonin?

(A) Meals consisting almost exclusively of protein

(B) Meals consisting almost exclusively of

carbohydrates

(C) Meals that would elicit insulin secretion

(D) Meals that had very low concentrations of tyrosine

(E) Meals that had very low concentrations of leucine