PrepTest 156

[lcid:3721] Prep Test 156 LSAT — Logical Reasoning — S2 Logical reasoning

Question prompt

Geologist: As a spinning Remaining source text redacted.
Why the credited answer is right

Credited answer: B

The notes below walk through why it fits the stem and how to eliminate the rest.

Argument or Facts

Argument

Valid or Flawed

Flawed

Strategy Overview

Argument or facts? Always argument, so identify premises and conclusions of argument Anticipate why the premises are not enough to show that the conclusion is true
Causal argument? Look for answer choice that:
Shows possible alternate cause Shows cause without effect Shows effect without cause Shows the cause and effect are reversed
If not causal argument, anticipate ways to worsen problem with argument
Use anticipations to select the answer choice that, if true, would weaken argument

Answer Anticipation

Our ultimate goal is to find the answer choice that weakens the argument. However, it's much easier to do that if we first break down the argument and identify its flaws. Then, weakening the argument is only a matter of finding the answer choice that exploits that flaw. Causation flaws are rampant on Weaken questions, and this argument is definitely causal. Note that the geologist explains why something is happening. Explanations are inherently causal — you're proposing a cause for an observed effect. In the geologist's argument, the cause is that "tectonic plate movements are making Earth's radius smaller," and the observed effect is that "Earth's rate of rotation increased" over the past decade. The correct answer could weaken this relationship. When an argument identifies a cause of an observed phenomenon, the correct answer typically proposes an alternative cause or shows an instance of the effect appearing without the cause. In the former case, the correct answer will present another plausible explanation of the Earth's faster rotation. In the latter case, the correct answer will show that the effect can occur or is occurring without the supposed cause. So, the correct answer could show that the Earth is rotating faster even though tectonic plate movements aren't decreasing Earth's radius. Although it's less likely, the correct answer could instead present an instance of the cause appearing without the effect — other times when tectonic plate movements decreased Earth's radius without the Earth rotating more rapidly. One of these three cause-and-effect weakeners — alternative cause, effect without the cause, cause without the effect — will probably appear in the correct answer. Still, many things could feasibly weaken this argument. Let's look for one of these cause-and-effect weakeners, but we'll keep an open mind while doing so. We'll start our analysis of each answer choice by asking, "Does this make it less likely that the Earth is rotating faster because tectonic plate movements are making Earth's radius smaller?"

Answer choices

  1. A
    In the beginning of Remaining source text redacted.
    Why choice A is not credited

    This is consistent with the geologist's explanation. The geologist proposes that earthquakes move tectonic plates and make Earth's radius smaller, increasing how quickly Earth is rotating. Given that, we'd expect earthquakes to consistently make Earth's radius smaller. This answer establishes that earthquakes made Earth's radius smaller at the beginning of the twentieth century. If it happened back then, we're more likely to accept that it's happening now. So, rather than weaken the geologist's argument, (A) strengthens it (slightly).

  2. B
    During the past decade, Remaining source text redacted.
    Why choice B matches the stem

    This weakens the geologist's argument by providing a sneaky example of "effect without the cause." This answer choice shows that other events have "counteracted" the movements of tectonic plates in the last decade. The geologist posits that tectonic plate movement has decreased Earth's radius, and this decreased radius explains Earth's faster rotation over the last decade. But if the plates' movements have been counteracted by other events, then the movements are not, in fact, making Earth's radius smaller. So, the Earth is spinning faster (effect ...), but the Earth's radius is not becoming smaller (... without the cause). If the Earth's radius isn't becoming smaller, something else must be causing the Earth's rotation to increase.

    Because this is one of the common cause-and-effect weakeners we anticipated, we can select (B) and give the remaining options an, at most, cursory reading.

  3. C
    Only skillful figure skaters Remaining source text redacted.
    Why choice C is not credited

    This is totally irrelevant to the geologist's explanation. The part about figure skaters is just meant to illustrate that an object with a smaller radius will rotate faster. Even if you have to be a "skillful" figure skater to bring your arms in while twirling, it can still be true that objects with smaller radii tend to spin faster.

  4. D
    Since the time of Remaining source text redacted.
    Why choice D is not credited

    This is consistent with the geologist's explanation. The Earth's rate of rotation may historically fluctuate when the Earth's radius increases and decreases! If we knew that Earth's radius has remained constant (until this decade) but Earth's rate of rotation has always fluctuated, this would definitely weaken the geologist's explanation. In that case, the answer choice would provide many instances of the effect (rate of rotation increasing) without the cause (radius decreasing). But without telling us about the Earth's radius since the time of the ancient Egyptians, this answer choice doesn't weaken the argument.

  5. E
    Increased volcanic activity over Remaining source text redacted.
    Why choice E is not credited

    This is also consistent with the geologist's explanation, but it looks like an alternative cause to many test-takers. However, the answer choice doesn't say that volcanic activity is causing the Earth's tectonic plates to move. If the answer choice said that, it would weaken the geologist's explanation (at least the part about earthquakes causing tectonic plate movement). However, as this answer choice is written, the volcanic activity could be an effect of the tectonic plate movement caused by earthquakes. If so, this would confirm that tectonic plates have been moving more in the last decade and that this may lead to the Earth's increased rate of rotation.

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