The extremely high incidence of birth defects and cancer among children in Tellersville, Nevada, has long been suspected to be linked to the nuclear-weapons facility in which nearly three-quarters of the town’s population works. It now appears, however, that the town’s water supply, tainted by the discharges of a nearby plastics factory, is to blame for the town’s health problems. Scientists have studied two groups of rodents. One group was exposed to the same level of radiation present at the nuclear-weapons facility but given pure water. The other was exposed to no radiation but given tainted water from Tellersville. The latter group showed an incidence of cancer and birth defects ten times higher than normal and six times higher than that of the pure water group.

Which one of the following, if true, most undermines the conclusion that Tellersville’s water supply is responsible for the town’s health problems?


A) Nuclear radiation is not a cause of birth defects and cancer among humans.

B) While radiation tends to affect all mammals in similar ways, the toxins present in Tellersville’s water supply have very different effects on rodents or other mammals.

C) Water from a new source would help Tellersville prevent future health risks.

D) Long-term exposure to toxins in the water supply in combination with exposure to radiation may significantly increase the incidence of birth defects and cancer among children.

E) Certain toxins in the water supply of Tellersville are likely to have served to protect those drinking it from some of the harmful effects of radiation.

OA: B.

I got this question from Princeton Review online sets. I chose B as the answer earlier but then i thought this could not be the right one because the explanation which I found in the answer says "The researcher's conclusion is based on the assumption that the rodents' reaction to the impurities in the water are somehow equivalent to the humans' reaction. (The "new stuff" in the conclusion is the humans). To weaken the argument, attack this assumption. Answer choice B is the only one which mentions a difference in reactions between the rats and the people.". But aren't this answer itself wrongly assuming that the effect of drinking toxic water is more on rodents than other mammals (say town people). Since the option B says the effect differs from rodents to mammals, but it may be more on other mammals also.

Please correct me if I'm wrong. The next best option which seemed to me is E.

You're missing an essential point in the structure of these questions. We are asked: which of the following, if true ....

As a result, we are ALLOWED to assume that this is possible. Basically, we can take it as a new premise, one that happens to expose/disprove the assumption that you already discussed.

I believe the answer is D. Option B does not exactly clarify whether the effects will be less severe or more severe in mammals. Since pure water group also shows higher incidence, it could be that both the factors are responsible and not the water supply alone. The pure water-radiation shows higher incidence, so radiation factor cannot be ruled out.

Could any expert give his opinion?

Here you are comparing whether C(toxin) has a higher effect on A(health) rather than B(nuclear). Actually what you have stated " Option B does not exactly clarify whether the effects will be less severe or more severe in mammals.", precisely tells us why this will be a weakener to the argument. If it is not easy to predict based on the rodent test that humans will also have similar reaction to C and B causes , how one claim that C will have a desired effect on humans. What if C is less harmful to humans and B is more? While for rodents its the other way round.

So this choice basically hits the conclusion, making it weaker.

That's a very nice explanation rishisbook

Thanks, rishisbook. A conclusion is something we are pretty sure will happen. To undermine it, we just need to introduce some doubt, not necessarily prove that the opposite will happen. In this case, we break the causal link by showing that there is some uncertainty about whether we can apply the case of the rodents to that of humans..

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Tim Sanders
Manhattan GMAT Instructor