PrepTest 124

Incorrect. The conclusion isn't about soil health or the relative importance of the different functions of earthworms, so this answer is out of scope.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with Necessary Premise

Stimulus Summary:
Earthworms like neutral soil (acid-to-alkaline scale). Soil is naturally acidic (decomposing plants), and crushed limestone is alkaline. So adding crushed limestone to soil will make worms like it more.

Answer Anticipation:
First, note what we didn't include in our Summary—the information that earthworms are essential for soil health. Since the conclusion, nor any other premise, is about soil health, that's background/a throwaway line there meant to distract.

Next, let's start with the conclusion and work backwards to find where the gap in the argument is. To the assumption of the argument, we should see how the terms in the conclusion are connected to the premises, and what link is missing.

The conclusion here states that adding crushed limestone to soil should make earthworms like it better. What is established about crushed limestone? It's alkaline. What's established about soil in the premises? It's acidic. What's established about earthworm preferences in the stimulus? They like neutral soil. So, rephrased, this conclusion states: Adding alkaline crushed limestone to acidic soil should make it more neutral.

Is there any indication that's how this process works? Not really. You don't need to know science for the LSAT, so don't start thinking about your chemistry classes! Instead, look at the argument—does it ever establish what happens when you add an acid and alkaline substances? Does it ever talk about what effect crushed limestone has when added to soil? Nope! There's no indication that this soft recommendation would work.

The correct answer, therefore, should establish that adding crushed limestone to acidic soil can make it more neutral and thus more attractive to earthworms.

Answer Explanation:
This answer discusses the effect of adding crushed limestone to soil, stating that it can neutralize some of the natural acidity in the soil. If the crushed limestone doesn't stay in the soil long enough to neutralize the acidity, then the recommendation in the conclusion makes no sense. Therefore, this answer is necessary for the argument to work.

Key Takeaway:
Sometimes, the correct answer in a Strengthen with Necessary Assumption question doesn't address the entirety of the assumption, but rather one aspect of it. Here, the assumption was that crushed limestone, when added to soil, neutralizes some of the acid. The correct answer only dealt with it persisting in the soil—not with other aspects of neutralization. That's fine! Any component of the assumption is necessary for the argument to work.

This most commonly expresses itself as eliminating a single alternative explanation or cause in an argument that concludes a specific explanation/cause.

Incorrect. This answer doesn't address the acid levels of the soil at all, which was the key to the conclusion. Additionally, it does address the soil health, which we determined to be out of the scope of the conclusion.

Incorrect. This answer rules out the possibility of a positive feedback loop for the soil—the acidity makes plants decompose faster, which increases acidity, which increases plant decomposition . . . This possibility was neither mentioned nor relied on by the argument, so it's out of scope.

Incorrect. This answer is about the benefit of earthworms to soil, not the attractiveness of that soil to earthworms, so it's not necessary for the argument.