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

Seth on November 23, 2015


Why is the answer D and not E?


Mehran on November 24, 2015

Thank you for your question. This is an Argument Completion logical reasoning question, a rare sub-type of Must Be True questions. Notice that the stimulus consists of a series of facts. The question stem is asking us to select the answer choice which appropriately (logically) completes the information set forth in the stimulus.

Let's look at the facts carefully. We are told that (1) fluoride enters groundwater when rain dissolves fluoride-bearing minerals in the soil, and (2) if 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.

Answer choice (D) — that sodium in groundwater INCREASES the rate at which fluoride-bearing minerals dissolve — can be reasonably concluded on the basis of the facts set out in the stimulus. When all other things are held constant, the concentration of fluoride in groundwater is significantly higher in those areas where the water contains a lot of sodium. When there is a lot of sodium, there is also going to be a higher concentration of fluoride.

Answer choice (E) establishes a correlation between high concentrations of sodium and fluoride in soil, but the stimulus doesn't give us information about soli concentrations — only about groundwater. So (E) can be eliminated.

Check out or review the Argument Completion lecture / example in the LSATMax app, and then let us know if you have any additional questions.

Seth on November 24, 2015

Thank you for your help

Niloo on September 29, 2021

So I understand via the explanation: the problem and why the correct answer choice is what it is. However, can someone just please explain to me why what answer choice E says is PROBLEMATIC, given that a correlation of high fluoride concentration to a high concentration of sodium does exist and is established by the stimulus. Like the explanation says that E reverses the correct relationship and thus, incorrectly states it, but I don’t necessarily understand why it is wrong to say that If A is associated with B, then B is associated with A. Does my question make sense?

Ravi on February 5 at 11:33AM

E doesn't work because we don't know anything about sodium-bearing minerals. The sodium that's in groundwater could originate from something else. Additionally, more sodium means more fluoride when other variables (such as the concentration of fluoride-bearing minerals) are held constant, so we can get rid of this choice.