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.

Large quantites of the sweetener corresponds to the high level of amino acid in the blood (if true).

My comment on E):
It's kind of standard to conduct experiments with one controll group whom receives placebo-pills/"sugar pills".

The key is in the words : large quantities. the solution was fairly simple ...
_________________

B.

I don't see a rationale for E. The question asks, how does the sweetner CAUSE the effect.

E has no cause, it merely explains the experiment procedure. As it has overtones of avoiding the placebo affect, it is a natural draw, but the question very clearly asks for a cause.

E is almost like choosing an option that says "both groups were given different drugs", ok, but so what? There is no causal link there that the question asks for.

Posted from my mobile device

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.

The conclusion: The detrimental effects were attributed to an amino acid that is one of the sweetener’s principal constituents. shows the cause-effect relationship: An amino acid in the sweetener ---> detrimental effects (lower cognitive ability)

I think it is a Strengthen, not an Assumption question.

Choice E does not strengthen the conclusion. The fact that subjects know what they are consuming or not does not shed any light about the cause-effect relationship above.
_________________

Answer B.
It provides the missing link and makes the conclusion more believable.

I chose B, as it gives the reason.

Although I have doubt- is option E an assumption?

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
_________________

Dear moulayabdesslam,

My friend, I should have noticed this before my previous response, but you committed a breach of protocol on GMAT Club. Never never never post in a new thread a question that has already been posted. Many GMAT Prep questions are already here. Always always always search extensively for a question before posting it in a new thread. If a question has already been posted, ask your questions there (you may find your question has already been answered by the experts' responses in that pre-existing thread). This particularly question has already been posted, so I merged your topic into this one. If you wish for more kudos, the first step would be to learn all the protocols of GMAT Club and not to violate them.

Does all this make sense?
Mike
_________________

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

--
Need Kudos

A) Most consumers of the sweetener do not consume as much of it as the experiment group members did.
Irrelevant. We are only concerned with members of test groups.

B) The amino acid referred to in the conclusion is a component of all proteins, some of which must be consumed for adequate nutrition.
This would weaken the researchers' conclusions.

C) The quantity of the sweetener consumed by individuals in the experimental group is considered safe by federal food regulators.
Also would weaken the researchers' conclusion. If a third-party has already done research, there would likely be another cause. Obviously not impossible that the researchers come to a different conclusion, but this answer requires too many assumptions for the GMAT.

D) The two groups of subjects were evenly matched with regard to cognitive abilities prior to the experiment.
Strengthens. Minimizes the chance of a third cause. In research experiments, the more similar the test group and the control group are the stronger the conclusion(s) of the study.

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.
Irrelevant. We're only concerned about the first experiment.

+1 KUDOS IF EXPLANATION WAS HELPFUL

(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

Para says There are 2 groups One group had loads of a sweetener and the other didn't. And the abilities of the group which had the sweetener decreased after the intake.

Ques : Pick an answer that proves sweetener is at fault.

Hence D. Earlier there were on same level, then some had sweetener and lost there abilities
_________________

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

Hello from the GMAT Club VerbalBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________