Dental researchers recently discovered that tooth-brushes can become

I have chosen the answer C for this question:

The underlying assumption in the passage is that contamination with bacteria can cause illness among individuals. Therefore, we should be switching toothbrushes once a month to avoid this.

A. The fact that dental researchers could not discover why contamination happens does not matter because the contamination still happens and can cause illness. Therefore, individuals should still replace their toothbrushes. This answer choice does not weaken the argument.

B. This is irrelevant - we do not care that dental researchers did not investigate other types of contamination.

C. This is the correct answer. If individuals who use toothbrushes that are contaminated do not get any sicker than individuals who use uncontaminated toothbrushes, then changing the toothbrush every month is unnecessary.

D. This somewhat supports the argument because careful cleaning of the toothbrush does nothing to eliminate the contamination, so people should be continually changing toothbrushes.

E. The fact that the amount of bacteria did not increase does not weaken the argument. The reason is because there is still a certain amount of bacteria on the toothbrush that could cause illness to individuals.

Bumping for review and further discussion*.

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This is a good causal problem

Contaminated brush (X) caused pneumonia and step throat (Y) . One way to weaken the causal issue is

Even if cause does occurs effect occurs (No X ---> still Y)
Ans C says that even uncontaminated brush cased pneumonia and step throat (Y)

Therefore C is clear answer

Conclusion: people should replace toothbrush at least once a month
Premise: Toothbrush gets contaminated with bacteria after 4 weeks of use.

Assumption: bacterial is more likely to cause illness

answer choice will undermine this assumption

Alternatively: Negate conclusion and select answer choice that supports the negated conclusion along with the premise.
Negated Conclusion: people should not replace toothbrush at least in a month.

Dental researchers recently discovered that tooth-brushes can become contaminated with bacterial that cause pneumonia and strep throat. They found that contamination usually occurs after toothbrushes have been used for four weeks. For that reason, people should replace their toothbrushes at least once a month.

Type - weaken
Boil it down - Contamination of toothbrushes occur after four weeks of usage and hence people should replace their toothbrushes once a month.
Pre-thinking - to weaken this we need an option that tells that probability that people will contract pneumonia and strep throat is minimal even after first four weeks

(A) The dental researchers could not discover why toothbrush contamination usually occurred only after toothbrushes had been used for four weeks. - Irrelevant - reason is not important

(B) The dental researchers failed to investigate contamination of toothbrushes by viruses, yeasts, and other pathogenic microorganisms. - Out of scope

(C) The dental researchers found that among people who used toothbrushes contaminated with bacteria that cause pneumonia and strep throat, the incidence of these diseases was no higher than among people who used uncontaminated toothbrushes. - Correct - If this is true then there is no point replacing the toothbrush every 4 weeks

(D) The dental researchers found that people who rinsed their toothbrushes thoroughly in hot water after each use were as likely to have contaminated toothbrushes as were people who only rinsed their toothbrushes hurriedly in cold water after each use. -Irrelevant

(E) The dental researchers found that, after six weeks of use, greater length of use of a toothbrush did not correlate with a higher number of bacterial being present. - Incorrect - what about the period of time from 4 weeks to 6 weeks .

Answer C

Question Que: "weaken the conclusion"
Question Type: weaken, argument

To weaken this type of question:
1. find conclusion 2.find evidence (premise) 3.find assumption (gap) 4. ATTACK the gap (alternate cause)

Conclusion: Replace contaminated toothbursh after a month
(Scope change)
Evidence: Used Tooth brush after a month are contaminated with bac p and s

Assumption (pre-thinking that closes the gap):
If Use Contaminated toothbursh, then will be infected with P and S, therefore must throw toothbursh

Weaken:
Using contaminated toothbursh might not get infected with P and S. It might be Other reason that cause the infection.

Answer Choice C:
The incedence of diseases using contaminated and non contaminated are the same. (Might something else that causes the infection)

I think something wrong with c. because it implies that using a contaminated toothbrush does not have any bad impact and does not cause disease, however, one of the premises says that bacteria causes disease!!!

No, C says: the incidence of these diseases among people used toothbrushes contaminated with bacterial is equal or less than incidence of these diseases among people who used uncontaminated toothbrushes.

means when they found results of tooth brushes which were uncontaminated , they were similar to tooth brishes which were contaminated.

so it contradicts the following premise, doesn't it?!
Dental researchers recently discovered that tooth-brushes can become contaminated with bacterial that cause pneumonia and strep throat.

if they keep other circumstances the same in that experiment and just examine the effect of contaminated brushes with bacteria, then the incidence of
the disease must be higher among people who use contaminated brushes. because the bacteria definitely causes pneumonia and strep throat!!!

The premise didn't say : Dental researchers recently discovered that tooth-brushes can become contaminated with bacterial that cause pneumonia and strep throat MORE THAN UNCONTAMINATED TOOTH BRUSHES.
Why would you assume comparison when no such statement is given in argument.

If I say : You are rich. Next I say : He is richer than you. It does not mean you are poor.

Similarly C says uncontaminated brushes have more or equal chances that can cause pneumonia and strep throat .

Can somebody explain what exactly is the conclusion here?

Accroding to option E, after 6 weeks of use the no. of bacterial does not increase even if one keeps using the same toothbrush. It implies that once the toothbrush gets contaminated with bacteria after 4 weeks, the no. of bacterial increases between the 4-week to 6-week window., BUT not beyond that window.

However, the argument is not concerned with how the no. of bacterial changes between 4-week to 6-week window and beyond the window. It is only concerend with (or rather suggests) what action needs to be taken once the toothbrush gets contaminated after 4 week of use to prevent pneumonia or strep throat.

Therefore, option E does not even address the core issue (or conclusion) of the argument. Therefore, it is a wrong answer.

GMATNinja, KarishmaB

I am confused between options C and E..Please explain what's wrong with option C.
It's saying that people whose toothbrushes are contaminated with bacteria - this is equal to people who use uncontaminated toothbrushes.
If people who are not using contaminated toothbrushes are equally at risk then what's the point of changing the toothbrush once a month?

Please help.

I think you might have misread the answer -- there's nothing wrong with (C) because it's the correct answer to this question!

Your reasoning for (C) is right. People with and without dirty toothbrushes are equally at risk -- and that removes the main reason for replacing toothbrushes. That's why (C) weakens the author's conclusion.

(E), on the other hand, says that 4-week old and 6-week old toothbrushes are equally dirty. But this just means 6-week old toothbrushes should be replaced too (ideally two weeks earlier).

That's why (E) doesn't weaken the argument.

I hope that helps!