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PrepTest 73, Logical Reasoning 1, Question 20


Question 20, when a question asked what assumption the argument relies upon, then it is a necessary assumption question. Which means that we have to find something that the argument hasn't said so far, that it requires in order for its evidence to prove its conclusion. Necessary assumptions are things that if they're not true, the evidence won't prove the conclusion.

The argument will fall apart. In order to look for them, we have to understand the argument, which means understanding the conclusion and the evidence. Here the conclusion comes at the end, helpfully flagged by the so. So to significantly improve soil structure, farmers will need to abandon the use of chemical fertilizers.

Of course, we need the evidence to know what they're talking about. The first sentence gives us a setup. It tells us something that happened. So there was a region where they used to grow green manure crops like alfalfa in order to regenerate their soil. And the advent of chemical fertilizers led them to stop doing that.

And then as a result, the soil structure in the area went down. So switching to chemical fertilizers led them to move away from green manure and that ruin the soil. So they have to get rid of the chemical fertilizers in order to get the soil structure back. Now this argument has a fairly common structure.

There was something that happened. There was a bad result, and in order to fix the bad result, we have to undo the thing that happened. But that's not always true of every bad thing. If you get stubbed by a knife, yeah, you do have to remove the knife in order to heal your wound.

But if someone moves into your apartment and starts living on your sofa and then you don't have a place to watch TV, well you might still be able to find some way to watch TV even if you don't move them from the sofa. Sometimes problems are mutually exclusive with solutions and sometimes they're not. For this argument to work, they need to be mutually exclusive. The fertilizers need to stop us from being able to enact the solution.

So the assumption is that chemical fertilizers prevent the growing of green manure. So let's go to the answer choices looking for something that says that. Now answer choice A, is tempting but wrong because it focuses on alfalfa. The argument doesn't require us to use alfalfa, they could use anything that does the same thing.

So just an individual example is not required by the argument in any way. We'll go on to answer choice B. Now this answer might be tempting because at the very least, it does put chemical fertilizers and green manure crops at odds with each other by saying that fertilizers have no positive effect on alfalfa growth. But actually, this is not an opposition that we need.

We don't need green manure to have no benefit from having fertilizers around. We need the fertilizers to block the use of green manure. So answer choice B is not our answer. Answer choice C, like with a lot of answer choices that bring up the most important thing, this answer isn't necessary. We don't need soil structures would be the most important thing for running your farm.

We just need soil structure to require getting rid of chemical fertilizers. There could be other more important things for soil quality, and that wouldn't affect the argument either way. So we don't need the soil structure to be the most important thing. Now answer choice D, this is another close but not quite. We don't need the chemical fertilizers to be responsible for the damage to the soil structure just to be preventing the soil structure from getting better.

So it could be that the soil structure was destroyed by something else, and the fertilizers just keep us from fixing it. We don't need the fertilizers to be the ones damaging it. Thus, it means that our answer is answer choice E. Let's see why. Many if not all farmers in the region will not grow green manure crops unless they abandon the use of chemical fertilizers.

This is what we wanted because it puts the green manure crops in opposition to the chemical fertilizers. They won't do the green manure unless they get rid of the chemical fertilizers. So answer choice E is what we were looking for, it is our answer.

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