Naturalist: For several years, coyote have had a robust population

For several years, coyote have had a robust population. (doesn't say population has declined now)
Over the past 7 years, CCC has been releasing low levels of bromide compounds into the air and groundwater. (note, low levels)
Some of these compounds, in sufficient concentrations, are suspected of compromising the immune systems of young coyote. (note, in sufficient concentrations)
A recent study over the entire forest established that 30% of young coyote did not survive their first year of life.
Conclusion: the bromide compounds released by Canic pose a direct threat to the well-being of coyote in the forest.

The argument doesn't say that the population is reducing. It suspects that sufficient concentration could reduce immunity of young. Then we are told that 30% of the young do not survive. But here is the important question - what does this 30% mean? Is it a number of concern? What if 30% coyote young do not survive naturally too? Then the chemicals may not be a problem. What if only 5% coyote young die of natural causes? Then it seems too many are dying and something (perhaps the chemicals) is at play here.
To evaluate the conclusion, we need to know what percentage of young survive naturally (without the chemicals in their environment). Animals give birth in litters and often only a fraction of their young survive. We need to know what that fraction is for coyotes.

Option (E) What percentage of healthy coyote survive to adulthood in the wild?
Gives us what we were looking for. If 95% healthy coyote survive to adulthood in the wild, then the chemicals seem to be a threat.
If 70% healthy coyote survive to adulthood in the wild, then the chemicals seem to be harmless.

I actually just glazed over the other options since what I was looking for was obvious. Had I not found it, I would have evaluated other options in this way:
A) Have significant levels of these bromide compounds been measured in the blood of other mammals in the forest?
Other animals are out of scope.

B) To what parts of the forest does the wind carry the particulate form of these bromide compounds?
Coyotes live throughout the forest. So this is irrelevant.

C) How many of the natural springs in the forest derive from groundwater that flows in the vicinity of Harrisville?
We don't know what this implies. We are given that the groundwater is contaminated. How many springs are there? Does flowing in the vicinity mean contamination but not otherwise? What if the contaminants percolate to all the groundwater? Which springs do coyotes use, if at all? There are too many unknowns to make any sense.

D) What naturally available nutrients would strengthen the immune system of young coyote against any effects of these bromide compounds?

Out of scope.

Answer (E)
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MAGOOSH OFFICIAL EXPLANATION:

The naturalist is clearly trying to sound an alarm, pointing to a problem that should concern us. The credited answer is (E). The big piece of evidence is: 30% of young coyote did not survive their first year of life. How does that percent compare to a normal natural rate? If, under the best, more environmentally pure, circumstance, some number close to 30% of first-year coyotes don't survive, then what's happening in this forest is not all that different than what usually happens, and the naturalist seems be making a great deal of chin music for nothing. If a much lower percent of first-year coyotes typically dies, and 30% is a unusually high number, then that underscores the urgency of the naturalist's claim. The answer to this question has strong implications for the strength of the argument, so this is a very important point to evaluate.

Choice (A) is suggestive. This argument is about the coyote, so what is happening with other mammals is only marginally relevant. Choice (A) is incorrect.

The argument concerns coyote over the whole forest. Even if wild-blown bromide compounds do no reach some parts of the forest, the bromide compounds in the groundwater may still reach those places. Knowing the answer to this question would still leave other important questions open, so it's not a crucial evaluation question. Choice (B) is incorrect.

Choice (C) is phrased in a funny way. Suppose we knew the answer --- suppose, say, someone with data about the region could tell us: exactly 15 springs in the forest derive from groundwater that flows in the vicinity of Harrisville. If we knew that number, then what? We still wouldn't be able to evaluate the argument. Choice (C) is incorrect.

IF the bromine compounds really pose a problem, then the question in choice (D) would be important for solving the problem. The question we need to answer first, though, is: is the argument correct? is the naturalist right? do the bromine compounds really pose a problem? Before we can solve the problem, we need to know that there is a problem! Choice (D) does not address the more fundamental question: is the naturalist correct? It does not help us evaluate the naturalist's argument.

Hi
How do you apply variance test to this question to narrow down the correct answer choice?

GMATNinja VeritasKarishma

i would go with option E as it appears to evaluate the argument properly.It helps in understanding whether bromide is affecting the adult population of coyote as well as or not

The naturalist's argument hinges on the recent study that shows 30% of young coyotes didn't make it to adulthood. While the suggested factor that results in this 30% was the release of the bromide compound, there is no reference point; we don't know if that number went up or, possibly, down. What if prior to the release of the compound, 50% of young coyote never made it into adulthood? Then it could be that the bromide is in some way helping the young coyotes survive.

The point is that E addresses a major issue in siding with the naturalist and it is an important piece of information to establish to properly test for the naturalist's reasoning.

I dont agree with E on the basis that 1st year of a coyote's life does not mean the coyote has become an adult yet. Lets say only 50% of coyotes survive to adulthood. welll, it might very well be that the second year of a young coyote will wipe out another 40% but we just dont know that.