Logical ReasoningDifficulty: Hard

PT4 S1 Q10 Explanation

Rhizobium bacteria living in the

A free, expert breakdown of this official LSAT Logical Reasoning question.

TopicsNecessary Assumption

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Stimulus

Rhizobium bacteria living in the roots of bean plants or other legumes produce fixed nitrogen, which is one of the essential plant nutrients and which for nonlegume crops, such as wheat, normally must be supplied by applications of nitrogen-based fertilizer. So if biotechnology succeeds in producing wheat bacteria, the need for artificial fertilizers will be reduced.

What this question is testing

Necessary Assumption

Conclusion

The author thinks: if biotech can engineer wheat to host Rhizobium bacteria the way legumes do, we won't need as much nitrogen fertilizer.

Evidence

The argument rests on what Rhizobium does in legumes — produces fixed nitrogen, an essential plant nutrient. The hope is that planting Rhizobium in wheat roots would do the same job.

Evaluate

Here's the gap. The author tells us Rhizobium produces fixed nitrogen in legume roots. But the argument's conclusion is about wheat roots. The argument quietly assumes that what Rhizobium does in legumes will carry over to wheat.

Negation test: imagine Rhizobium in wheat roots doesn't produce fixed nitrogen — maybe the bacteria's nitrogen-fixing activity depends on something legume-specific. Then engineering wheat to host the bacteria does nothing for the fertilizer problem. The conclusion falls apart, which is the test for a necessary assumption.

Goal

Pick the answer that says Rhizobium in wheat roots would produce fixed nitrogen.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question
10.

The argument above makes which one of the

Answer choices

  1. Bad Assumption2% picked this

    Biotechnology should be directed toward producing plants that do not require

    The argument doesn't need to claim biotech should be directed toward fertilizer-free plants — only that if it succeeds in producing Rhizobium-hosting wheat, fertilizer needs will drop. The author is making a conditional claim, not a recommendation about biotech priorities. Negation test: even if biotech shouldn't be directed this way, the conditional claim still holds.

  2. Bad Assumption15% picked this

    Fixed nitrogen is currently the only soil nutrient that must be supplied by artificial fertilizer

    "Currently the only" is too strong. The argument needs Rhizobium-produced fixed nitrogen to reduce fertilizer needs — that doesn't require fixed nitrogen to be the only nutrient currently supplied artificially. Even if other nutrients also need fertilizer, replacing the nitrogen-based fertilizer with biological nitrogen fixation would still produce a reduction. Negation test: if other nutrients also require fertilizer, the conclusion (some fertilizer reduction) still goes through.

  3. Bad Assumption10% picked this

    There are no naturally occurring strains of wheat or other grasses that have Rhizobium bacteria

    The argument doesn't need to claim no naturally occurring wheat strains host Rhizobium. It only claims that if biotech succeeds in producing such strains, fertilizer needs will drop. Whether nature already has such strains is irrelevant — the argument's conditional claim still holds either way. Negation test: even if some wheat naturally hosts Rhizobium, the biotech-conditional argument is unaffected.

  4. Bad Assumption1% picked this

    Legumes are currently the only crops that produce their own supply

    The argument doesn't require legumes to be the only self-fertilizing crops. The conclusion is about wheat specifically — that wheat strains hosting Rhizobium would need less fertilizer. Whether other non-legume crops also produce their own fixed nitrogen has no bearing on this. Negation test: if some other crops also produce fixed nitrogen on their own, the argument's wheat-specific conclusion is unaffected.

  5. Correct72% picked this

    Rhizobium bacteria living in the roots of wheat would produce

    Why this is right

    This is the assumption the argument needs. The author concludes that engineering wheat to host Rhizobium will reduce fertilizer use, but the only fact the author has cited is that Rhizobium produces fixed nitrogen in legume roots. The conclusion only follows if Rhizobium would also produce fixed nitrogen in wheat roots. Negation test: if Rhizobium in wheat roots would not produce fixed nitrogen, the bacteria contribute nothing, and there's no fertilizer reduction. The conclusion collapses — confirming this assumption is necessary.

    Skill tested: Necessary Assumption · how this choice captures the argument's function is the move to repeat next time.

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