73 LR Two Question 8
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I think miurella's answer below is a pretty good way of addressing it (and worth more than 0.02 cents!).
The broader answer is that the correct response must be proved by the passage. Here, there's nothing to indicate that (E) is necessarily true, so it cannot be the right answer.
Just my .02cents, and maybe Dave can correct, but I ruled out (E) because it was talking about soil, whereas the passage was referring to concentrations in the *groundwater*. Also, the stimulus doesn't let us infer anything about "sodium-bearing minerals" that (E) talks about. We only know about the high levels of *sodium* itself.
I approached this from a totally different perspective (well, not totally...maybe just slightly). I simply set this up as 2 equations:
FBM + rain = [fuoride in groundwater]
FBM + rain + high [sodium] = Î Î Î [fluoride in groundwater]
"D" seemed to fit the missing piece of the puzzle (almost like a resolution) question. it isn't perfect, because it introduces "rate", but one could follow that idea through to make the leap of "if it increases rate, it likely increases concentration".
*I eliminated "E" quickly bedause of the "held constant" status in the passage in line with my equation setup. To me, it seemed a bit out of scope and, but I absolutely agree with the previously-described trains of thought.
As I was writing this, I thought of a visual analog. Imagine adding water to tiny, tightly-ccompacted pebbles in a clear bucket with holes in the bottom, and collecting the liquid that seeps through. Let that progress until as much water gets filtered/colllected as possible (and record the amount of time taken for the experiment to finish). Repeat, but this time you add a bunch of large gravel with the small pebbles. You now find that you've collected more water in the same time than was observed in experiment 1. Again, not a perfect explanation, but one could imagine that the gravel has something to do with speed of filtration, so an increased "rate" reasonably/logically follows.