Weaken argument CRs: Attack assumption or Weakening Premise?

[GMATBLACKBELT]

When answering Weaken the Conclusion CR questions, is it preferable to choose the answer choice that attacks the assumption or introduces a piece of weakening evidence?

Here is a sample question:

Wind farms, which generate electricity using arrays of thousands of wind-powered
turbines, require vast expanses of open land. County X and County Y have similar
terrain, but the population density of County X is significantly higher than that of County
Y. Therefore, a wind farm proposed for one of the two counties should be built in
County Y rather than in County X.

Which of the following, if true, most seriously weakens the planner’s argument?

A. County X and County Y are adjacent to each other, and both are located in the
windiest area of the state.
B. The total population of County Y is substantially greater than that of County X.
C. Some of the electricity generated by wind farms in County Y would be purchased
by users outside the county.
D. Wind farms require more land per unit of electricity generated than does any other
type of electrical-generation facility.
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.



OA is C

[RonPurewal]

hi GBB -

Please cite the source of this question (and problem #, etc., if there is one).

I'll write more if you cite appropriately, but here's an extremely brief treatment:
Knocking down assumptions and presenting 'weakening evidence' are both valid. Upon closer examination, you will usually be able to find that one or the other isn't really happening (because otherwise there would be two answer choices).
A, C, D are irrelevant to the decision between X and Y.
B seems relevant, but isn't (it's population density and the presence of large tracts of open land, not a total head count, that matters).
E calls the conclusion into question, because it leads to the conclusion that county X may actually contain more 'vast expanses of open land' than does county Y, despite its greater overall population density.

[er.anjali]

This question is from GMAT prep.

[RonPurewal]

thanks for the source citation.

please re-post your question in the GMAT Prep folder. thank you.