When WEAKENING an argument, you must first find the LOGICAL FLAW: the missing assumption that would be needed to connect premises to conclusion.

Premises:
- Wind farms require vast expanses of open land.
- County X and County Y have similar terrain
- the population density of County X is significantly higher than that of County Y

Conclusion:
Therefore, a wind farm proposed for one of the two counties should be built in County Y rather than in County X.

Missing information:
- is "open land" directly correlated to "population density"? i.e. could there be high population density but also lots of open land?
- do we know that people are evenly spread out across the county? What if there's a dense city in just one part of the county, but lots of open land elsewhere?

If we want to WEAKEN, we need an answer choice that undermines the idea that population density & open land mean the same thing.

(A) County X and County Y are adjacent to each other, and both are located in the windiest area of the state.
This just reaffirms that the counties are otherwise comparable, but does not undermine the link between population density and open land.

(B) The total population of County Y is substantially greater than that of County X.
Both population density and proportion of open land are PROPORTIONAL metrics. Absolute population is irrelevant, as it does not undermine the link between population density and open land.

(C) Some of the electricity generated by wind farms in County Y would be purchased by users outside the county.
Irrelevant to whether pop. density = open land.

(D) Wind farms require more land per unit of electricity generated than does any other type of electrical-generation facility.
Irrelevant, because the only comparison the argument makes is about wind power and where to generate it.

(E) Nearly all of County X's population is concentrated in a small part of the county, while County Y's population is spread evenly throughout the country.
Correct! This undermines the idea that high population density in X = low proportion of open land. If most people live in own small part of the county, then presumably there is open land in the rest of the county.

The answer is E.

The logical flaw in this argument falls under the category of what I like to call MIXED METRICS: it seems like the premises and conclusion are talking about the same statistic, but in fact we're looking at different mathematical or categorical ideas.

Any time you see one statistic / metric / mathematical concept swapped in for another, ask yourself if there's a discrepancy between the two.

Here are several other examples along the same lines:
https://www.beatthegmat.com/the-differe ... tml#822399
https://www.beatthegmat.com/the-proport ... tml#820184
https://www.beatthegmat.com/critical-re ... tml#725229
https://www.beatthegmat.com/lyme-diseas ... tml#714758
https://www.beatthegmat.com/passenger-j ... tml#743749
https://www.beatthegmat.com/a-cr-less-a ... tml#776214
https://www.beatthegmat.com/statistics- ... tml#564609
https://www.beatthegmat.com/total-books ... tml#680834
https://www.beatthegmat.com/cr-evaluate ... tml#558393
https://www.beatthegmat.com/i-m-doubtin ... tml#551227