Half of the subjects in an experiment—the experimental group—consumed large quantities of a popular artificial sweetener. Afterward, this group showed lower cognitive abilities than did the other half of the subjects—the control group—who did not consume the sweetener. The detrimental effects were attributed to an amino acid that is one of the sweetener’s principal constituents.

Which of the following, if true, would best help explain how the sweetener might produce the observed effect?


(A) The government’s analysis of the artificial sweetener determined that it was sold in relatively pure form.

(B) A high level of the amino acid in the blood inhibits the synthesis of a substance required for normal brain functioning.

(C) Because the sweetener is used primarily as a food additive, adverse reactions to it are rarely noticed by consumers.

(D) The amino acid that is a constituent of the sweetener is also sold separately as a dietary supplement.

(E) Subjects in the experiment did not know whether they were consuming the sweetener or a second, harmless substance.



"Half of the subjects in an experiment" Strengthen Question

The key is in the words : large quantities. the solution was fairly simple ...
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B is correct.
The question asks a reason to explain why the sweetner caused lower cognitive ability?
Premise given is that an amino acid contained in the sweetner caused the detrimental effect.
High consumption of sweetner = high levels of amino acid because it's the principal constituent.
and B says that high levels of amino in blood causes brain malfunction. and Hence B is the answer.
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Answer B.
It provides the missing link and makes the conclusion more believable.
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Dear moulayabdesslam,
This is a great question! I'm happy to help.

For this and related question, it's good to have the general ideas of experimental design. See:
Medical Trials and the Placebo Effect on the GMAT Critical Reasoning

A. Most consumers of the sweetener do not consume as much of it as the experimental group members did.
The conclusion concerned the comparison of control group to experimental group. What people do in the outside world is not directly relevant to the conclusions based on this within-experiment comparison. This is not correct.

B. The amino acid referred to in the conclusion is a component of all proteins, some of which must be consumed for adequate nutrition.
That's fascinating but it doesn't help us at all in deciding whether this amino acid or anything else in the sweetener played a role in making the experimental group show lower cognitive abilities. This is not correct

C. The quantity of the sweetener consumed by individuals in the experimental group is considered safe by federal food regulators.
Well, that's certainly good: for legal and ethical reasons, we would hope that this is true, but other than keeping people alive, this plays no role in determining what caused the difference in abilities. This is not correct

D. The two groups of subjects were evenly matched with regard to cognitive abilities prior to the experiment.
Aha! This is interesting! Suppose this weren't true! For the sake of argument, suppose the challenge were mathematical in nature. Suppose the control group were a bunch of people with Ph.D.s in mathematics and experimental group were folks who could not get past Algebra One in school. Clearly, these two groups have differing mathematical abilities! If they perform differently on a math task, that's hardly a surprise, and probably not due to the sweetener or whatever is in it! If the two groups have different abilities, then that introduces a plausible alternative explanation for the differing performances in cognitive abilities. A good experimental design would guarantee that the folks in the two groups were evenly matched in cognitive skills, so that there was no pronounced difference in either direction, so that the result can only be attributable to the sweetener and nothing but the sweetener. This extraordinarily strengthens the conclusion!

E. A second experiment in which subjects consumed large quantities of the sweetener lacked a control group of subjects who were not given the sweetener.
The second experiment sounds poorly designed, and tells us nothing about the conclusion in the first experiment. This is incorrect.

The only possible answer is (D).

Let me know if you have any questions.

Mike
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Thank you, Mike, for a wonderful explanation! The Ph. D/Algebra One Class analogy made me laugh. As a friend says, rien de mieux que de joindre l'utilie à l'agréable! (Tr: Nothing is better than to join the useful to the pleasant).

Have a great evening!
Moulay

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Need Kudos
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(A) Most consumers of the sweetener do not consume as much of it as the experimental group members did. - Incorrect. Out of scope.

(B) The amino acid referred to in the conclusion is a component of all proteins, some of which must be consumed for adequate nutrition. - Incorrect. Out of scope.

(C) The quantity of the sweetener consumed by individuals in the experimental group is considered safe by federal food regulators. - Incorrect. Irrelevant.

(D) The two groups of subjects were evenly matched with regard to cognitive abilities prior to the experiment. - Correct. Supports the claim mentioned in the argument by stating that there was no disparity between the subjects chosen.

(E) A second experiment in which subjects consumed large quantities of the sweetener lacked a control group of subjects who were not given the sweetener. - Incorrect. Out of context.

Answer: D

The question is a strengthen question in earnest.

There is a logical correlation and causation gap that exist when you try to link the premise and the conclusion.

Anything that explains why amino acid in the conclusion inhibits cognitive function would add more credibility to the conclusion and hence strengthen it.

B does exactly that.

Hope this helps.

A Kudos for my worries would be appreciated :D