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Difficulty: HardIdentifying Necessary Assumptions

Environmentalist: Two years ago, the city implemented a ban on the use of artificial fertilizers in all residential gardens to improve the water quality of the adjacent Lake Azure. Recently, tests showed that the concentration of toxic algae in Lake Azure, which feeds on synthetic nitrogen, has decreased by 40%. Therefore, the city's ban successfully achieved its intended goal.

Which of the following must be assumed in order for the environmentalist's conclusion to be properly drawn? (Select all that apply)

  1. The artificial fertilizers previously used in the city's residential gardens contained synthetic nitrogen.Answer
  2. A reduction in the concentration of toxic algae contributes to an improvement in the water quality of Lake Azure.Answer
  3. C
    The ban on artificial fertilizers was the sole cause of the 40% decrease in the concentration of toxic algae.
  4. D
    The water quality of Lake Azure was worse two years ago than it had been at any point in the previous decade.
  5. E
    Any reduction in the use of artificial fertilizers near a body of water will inevitably lead to an improvement in its water quality.

Answer

The correct assumptions are that the fertilizers contained synthetic nitrogen and that a reduction in toxic algae contributes to an improvement in water quality.
The argument concludes that the ban achieved its intended goal (improving water quality) based on the evidence that toxic algae decreased. For this reasoning to hold, two unstated premises are strictly necessary. First, there must be a factual link between the banned item (artificial fertilizers) and the algae's food source (synthetic nitrogen). Second, there must be a logical link between the evidence provided (less toxic algae) and the stated goal (improved water quality). Without both of these links, the argument collapses.

Step-by-Step Solution

1
Deconstruct the argument into its core components.
Premise: Ban on artificial fertilizers implemented to improve water quality. Premise: Toxic algae (which feeds on synthetic nitrogen) decreased by 40%. Conclusion: The ban achieved its intended goal.
Identifying the exact premises and the specific conclusion helps reveal logical leaps made by the author.
2
Identify language shifts between the premises and the conclusion.
Gap 1: The premise discusses 'artificial fertilizers' but then introduces 'synthetic nitrogen' as the algae's food. Gap 2: The evidence cites a decrease in 'toxic algae', but the conclusion claims an 'improvement in water quality'.
Necessary assumptions almost always exist to bridge unwarranted jumps in terminology or scope.
3
Test potential answers using the Negation Technique.
Negating the correct options destroys the argument. If fertilizers lacked synthetic nitrogen, banning them wouldn't starve the algae. If less algae doesn't mean better water, the goal wasn't achieved.
The Negation Technique definitively proves whether an assumption is strictly required by the argument.

Key Concept

Bridging Concept Gaps (Supporter Assumptions)
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Topics

Identifying Necessary Assumptions

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