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Difficulty: MediumWeaken

Agronomist: In a multi-year agricultural study across several commercial grain farms, fields planted with hairy vetch as a winter cover crop produced subsequent corn yields that were on average 20 percent higher than those from fields left fallow during the winter. Because hairy vetch is known to fix atmospheric nitrogen into the soil, this substantial increase in yield must be directly attributable to the additional nitrogen provided by the cover crop.

Which of the following statements, if true, most seriously weaken the agronomist's argument? Select all that apply.

  1. Fields planted with hairy vetch retained significantly higher soil moisture throughout early summer droughts than fallow fields, and moisture retention is the primary determinant of corn yields in these regions.Answer
  2. B
    Soil samples taken from the vetch-planted fields immediately before corn planting showed nitrate concentrations nearly double those of the fallow fields.
  3. Farmers participating in the study applied identical high levels of synthetic nitrogen fertilizer to all fields, raising soil nitrogen well above the maximum threshold usable by corn plants.Answer
  4. D
    The commercial farms in the study did not actually achieve any measurable increase in corn yield following the winter cover crop trials.
  5. E
    Several other leguminous cover crops fix atmospheric nitrogen at substantially higher rates per acre than hairy vetch.

Answer

The argument is weakened by the statement establishing soil moisture retention as an alternate cause for the higher yields, as well as the statement demonstrating that preexisting synthetic fertilizer levels already saturated the crops' nitrogen requirements.
The correct statements undermine the argument by challenging the causal attribution: one introduces an alternative causal factor (enhanced moisture retention during critical drought periods) that fully accounts for the yield disparity, while the other shows that nitrogen levels were already saturated beyond the threshold of utility, eliminating the possibility that supplemental nitrogen caused the yield improvement.

Step-by-Step Solution

1
Identify the conclusion and the supporting premises in the stimulus
Premise: Fields with hairy vetch cover crops produced 20 percent higher corn yields than fallow fields. Premise: Hairy vetch fixes atmospheric nitrogen. Conclusion: The yield increase must be directly attributable to the additional nitrogen.
Isolating the structural components reveals the core causal claim made by the author.
2
Identify the underlying assumptions and potential points of vulnerability
The author assumes that nitrogen fixation is the only relevant difference caused by the cover crop and that additional nitrogen was capable of driving yield gains in this setting.
Causal arguments can be weakened by offering an alternative cause for the effect or showing that the proposed cause could not have produced the effect.
3
Evaluate each option to find all statements that undermine the causal connection
Introducing moisture retention as the decisive factor provides an alternative explanation, while showing that nitrogen was already supplied past the crop's uptake capacity negates the proposed causal mechanism.
Both findings directly attack the link between the cover crop's nitrogen fixation and the observed yield increase.

Key Concept

Weakening causal arguments through alternative explanations and demonstrating the ineffectiveness of the proposed cause
Estimated Time:1m 30s
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