Most water companies

[akhp77]

Source: MGMAT CAT

Most water companies in the United States add fluoride to tap water to help prevent cavities. Some dentists argue, however, that this practice actually causes more harm than good because people overestimate the protection afforded by the fluoride and do not take the proper steps to care for their teeth, such as brushing and flossing after every meal. If water companies did not add fluoride, the dentists claim, people would be forced to be more active in their dental hygiene and tooth decay would decline as a result.

Which of the following, if true, most weakens the dentists' claims?

A: Fluoride is widely used in commercially available dental care products.
B: Most Americans are not aware that fluoride is added to tap water.
C: Annual visits to the dentist are the most effective means of controlling tooth decay.
D: The United States has the lowest rate of tooth decay in the world.
E: Most Americans already brush their teeth daily.

OA: B

Here, stimulus as a whole is valid only if Americans are aware that fluoride has added in to water. So B cut the assumption.

I could not understand option E. What is the explanation for E to reject it? IF Most Americans already brush their teeth daily, THEN argument has no affect.

Please explain.

[ashish2104]

@akhp77: You answered your question urself.
E has no effect on the argument or the dentist's claim.

You are asked to weaken the argument.

Dentist's assume that americans know that floride is added to the water and hence they can claim that water companies should not add flouride.

To weaken this, we simply need to show that people are not aware of flouride addition to water.

[akhp77]

@akhp77: You answered your question urself.
E has no effect on the argument or the dentist's claim.

What if I say like this?
If E is correct, Then dentists' claim does not play any role.
Is it irrelevant or weakening?

I would like to understand logical relation.

[tim]

akhp, you have gotten the cause and effect reversed. We are not supposed to investigate the effect the conclusion has on the answer choices, but rather the effect the answer choices have on the conclusion. The bottom line though is that E is irrelevant, which does not guarantee that it is incorrect (in theory an irrelevant choice could most weaken the argument if all the other choices strengthened it, but the GMAT does not do this in practice). However, since B actually does weaken the argument, E is thus ruled out..