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PrepTest 73, Logical Reasoning 2, Question 8


Question eight, this question ask what can be reasonably concluded from the information that we're given above? Which means that we're doing an inference question. And on an inference question, our job is to read the information, make sure we understand it and then select an answer that we can prove true. This passage gives us three pieces of information.

The first described how fluoride enters our region's groundwater. Apparently, it does. So when rain dissolves fluoride bearing minerals in the soil. The next two facts concern a study they did. They did a study, and in that study, they were careful to control it for all the variables other than what they talk about in the next part.

And this is important. When you hold all sorts of variables constant that means that you've factored them out of the experiment. They don't have any effect on the study. So the concentrations of fluoride-bearing minerals, rainfall, all those things are held exactly the same.

And when they did that, they found that fluoride concentrations in the groundwater are higher in areas where the groundwater also contains a high concentration of sodium. When you take everything else out of the picture, the more sodium that you have, the more fluoride you get in the groundwater. On the basis of those three facts, we're going to prove one of the answer choice's is true, but we won't know what we have to prove true until we look at the answers.

Answer choice A, this answer is definitely wrong. Fluoride-bearing minerals are not the primary source of fluoride in groundwater. We don't know what the primary source of fluoride in groundwater is. We just know that one way that fluoride gets into groundwater is when fluoride-bearing minerals are dissolved in the soil because of rainfall. So whether it's the primary source or just a source we don't know.

So we can't pick answer choice A. Now answer choice B, rainfall doesn't affect fluoride concentrations, that's actually been denied by the first sentence. We know that fluoride concentrations are affected by rainfall because that's what dissolves the fluoride bearing mineral. Answer choice B has been contradicted by the information.

Answer choice C, be careful. The passage never mentioned sodium-bearing minerals. He mentioned sodium, where that sodium came from, we don't know. Don't confuse the sodium with the fluoride-bearing minerals. This is completely new. We know nothing about it.

So answer choice D then, sodium and groundwater increases the rate in which fluoride-bearing minerals dissolve. Now this is supported. We know that when you have more sodium, you have higher fluoride concentrations. So a plausible way that that could work would be that the sodium is increasing the rate that the minerals dissolve.

Remember, they've held all the other variables constant, so nothing other than the sodium, could be affecting it. Answer choice D is supported by the information we've been given. It's our answer. What's wrong with E? E brings up the sodium very minerals that we don't know anything about.

So whatever it says here doesn't matter. We don't know anything about these sorts of minerals. So the answer is answer choice D.

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