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Difficulty: HardRuling Out Alternate Explanations (Defender)

Records from the coastal region of Veltierra show that over the last twenty years, the rate of structural failure among seawalls constructed using a newly introduced synthetic material, Duracrete, is significantly higher than the structural failure rate for seawalls built with traditional concrete during the exact same period. Based on these records, a civil engineering task force concluded that Duracrete is inherently less resilient to Veltierra's corrosive marine environment than traditional concrete is.

The task force's conclusion depends on which one of the following assumptions?

  1. A
    Any seawall built in Veltierra using a material less resilient than traditional concrete will experience structural failure within twenty years of construction.
  2. The seawalls constructed with Duracrete were not predominantly placed in areas of Veltierra's coast that are subjected to significantly harsher physical marine conditions than the areas where the traditional concrete seawalls were located.Answer
  3. C
    Traditional concrete seawalls in Veltierra are not subject to structural failure from factors entirely unrelated to the corrosive marine environment.
  4. D
    The overall number of seawalls constructed with Duracrete in Veltierra over the last twenty years did not greatly exceed the number of traditional concrete seawalls built during that time.
  5. E
    If a seawall material is inherently less resilient to a marine environment, it will always exhibit a higher rate of structural failure than more resilient materials.

Answer

The assumption that Duracrete seawalls were not subjected to harsher physical marine conditions than traditional concrete seawalls.
This option provides a classic Defender Assumption by ruling out a critical alternate explanation. The stimulus observes a correlation between a material (Duracrete) and a higher failure rate, concluding that the material is the cause. However, if the Duracrete seawalls were built in much harsher locations (e.g., facing open ocean storms while traditional walls were in sheltered bays), location would be the cause of the failures, not the material. By stating that this alternate scenario did NOT happen, this option defends the argument's causal claim. Using the Negation Technique: If the Duracrete seawalls *were* placed in significantly harsher conditions, the task force's conclusion falls apart.

Step-by-Step Solution

1
Identify the core argument's premises and conclusion.
Premise: Duracrete seawalls have a higher failure rate than traditional concrete seawalls over the last 20 years. Conclusion: Duracrete is inherently less resilient to the marine environment.
Understanding the exact structural gap between the evidence provided and the claim being made is the first step in evaluating any logical reasoning question.
2
Determine the type of argument and spot potential flaws.
This is a causal argument based on a correlation. It assumes that the material (Duracrete) is the sole cause of the higher failure rate.
Causal arguments based on observational data are always vulnerable to alternate explanations (third variables or confounders).
3
Anticipate necessary 'Defender' assumptions.
To protect the conclusion, the author must assume that other potential causes for the higher failure rate are not present. One major alternate cause would be if the Duracrete seawalls were subjected to rougher waves or worse weather.
A defender assumption works by actively eliminating a competing explanation that would otherwise destroy the argument's causal link.
4
Apply the Negation Technique to the best candidate.
Negating the correct option yields: 'Duracrete seawalls WERE predominantly placed in areas subjected to significantly harsher conditions.' If this is true, the harsher conditions explain the failures, destroying the conclusion that the material itself is to blame. Therefore, the original statement is a necessary assumption.
The Negation Technique is the definitive test for a Necessary Assumption. If the negated form of an option destroys the argument, that option is the correct answer.

Key Concept

Ruling Out Alternate Explanations (Defender)
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Ruling Out Alternate Explanations (Defender)

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