Fluoride enters a region's groundwater when rain dissolves fluoride-bearing minerals in the soil. In a recent study, ...

Mazen on May 17, 2022

Hi

This is of utmost importance, as it relates directly to the heart of understanding the logical structure in conditional principles/rules.

Lsatmax explanation for eliminating E is:
"Incorrect. High fluoride is correlated with high sodium levels, not the other way around. This answer reverses the relationship."

I respectfully and humbly disagree with this explanation, for I do not see an illegal reversal in E.

The word "where" introduces a sufficient condition!

E states:
"Soil that contains high concentrations of sodium-bearing minerals also contains high concentrations of fluoride-bearing minerals."

The second/last sentence of the argument states:
"In a recent study, researchers found that when rainfall, concentrations of fluoride-bearing minerals, and other relevant variables are held constant, fluoride concentrations in groundwater are significantly higher in areas where the groundwater also contains a high concentration of sodium."

When we abstract the logic from answer-choice E and the last sentence of the argument, we do not see an illegal reversal.

The issue with E, in my humble opinion, is that fluoride-bearing minerals are distinct from fluoride, and sodium-bearing minerals are also not the same as sodium.

In other words, the argument's last sentence draws a conditional rule between fluoride concentrations and sodium, whereas E draws a logically matching structure BUT between the "sodium-bearing minerals" and the fluoride-bearing minerals."

In retrospect, based on the argument, we can infer that a groundwater that contains a high concentration of sodium guarantees (or is sufficient to guarantee) significantly higher concentrations of fluoride in that very groundwater (the higher concentrations of fluoride in the same groundwater is the necessary part).

The flaw in E --"Soil that contains high concentrations of sodium-bearing minerals also contains high concentrations of fluoride-bearing minerals." -- is not that it commits an illegal reversal between the fluoride and the sodium.

The flaw is that E's sufficient part is "Solid that contains high concentrations of sodium-bearing minerals," and its necessary condition is "soil that contains high concentrations of fluoride-bearing minerals." And fluoride-bearing minerals and fluoride are not the same, and sodium-bearing minerals are also not the same as sodium.

Please let me know if I am wrong?

Thank you

Emil-Kunkin on May 21, 2022

Hi Mazen,
I think you are completely correct to draw a distinction between sodium and sodium-bearing minerals, and the same for fluoride. However, I do think that (E) does constitute an illegal reversal for a different reason. However, my approach probably would have dismissed (E) for an entirely different reason.

The stimulus tells us that when all other factors are constant, there is more fluoride in the groundwater when there is also a high concentration of sodium. This implies (D), it tells us that when sodium is present, fluoride is more likely to dissolve the floride bearing materials, leading to more fluoride in the groundwater.

(E) Tell us that Soil with a lot of Sodium-bearing materials also has a lot of fluorine-bearing minerals. The passage gives us some reason to think that the opposite is true- that when we have fluoride, we are also more likely to have sodium (although not necessarily their precursors). However, maybe sodium is extremely common, and fluoride is exceedingly rare. While the passage may leave one with the impression that fluoride is fairly common, we are actually told nothing about the relative commonness or rarity of sodium, fluoride, or their precursors.

Given this, how could we possibly know that one must follow the other? Surely we know nothing about how often soil contains either or both of the materials, and we are simply not able to make any claims about their relative likelihoods.

Ultimately your reason for eliminating E (Which, for the record, I do not at all disagree with), my reason, and stated illegal reversal reason (while I do not find particularly strong, but still is strong enough to eliminate E)all reflect the broader reason that (E) is incorrect: it is not even remotely supported by the passage. Wrong answer choices in LR (and reading comp to some extent) are often wrong for more than one reason, and this is a case where this is especially stark. While all of these reasons may at some level boil down to "the passage does not support E" there are certainly several angles to eliminate it.