Logical reasoning PrepTest 106 · Section 1 · Question 8

Question prompt

Generally speaking, if the Remaining source text redacted.
Why the credited answer is right

Credited answer: C

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

Argument or Facts

Facts

Valid or Flawed

NA

Question Type

Paradox Questions

Stimulus Summary

General rule - If the same crop is grown over and over again in the same place, it depletes nitrogen and grows worse
Exception? - Alfalfa increases the amount of nitrogen in the soil
Unexpected outcome - Alfalfa also grows worse when planted in the same place over and over

Answer Anticipation

As with all Paradox questions, we should start by clearly defining the paradox. This one is a little trickier, though! However, noting that we start with a general rule, and alfalfa is brought in as something that doesn’t perfectly align with that rule, gives us a place to start.
The general rule is that crops do worse when replanted in the same place because they use up the nitrogen. However, alfalfa “fixes” nitrogen - it adds it to the soil instead of using it up. Since alfalfa doesn’t run into the same problem as plants that use up the nitrogen and thus grow poorly, you might expect that alfalfa doesn’t end up growing worse when planted in the same place year after year.
Not so - it, too, grows less well in later years. Since this can’t be caused by a lack of nitrogen, it must be caused by something else, so let’s look for an answer establishing something alfalfa does that also contributes to it growing worse. Maybe too much nitrogen is just as much of a problem as not enough?

Answer choices

  1. A
    Some kinds of plants Remaining source text redacted.
    Why choice A is not credited
    Even if this applies to alfalfa, it doesn’t explain why that plant starts out doing fine but then grows worse after a few years - the stimulus doesn’t state that this happens only if other plants are removed from the same fields!
  2. B
    Alfalfa increases the amount Remaining source text redacted.
    Why choice B is not credited
    This answer explains how the alfalfa “fixes” nitrogen, but that’s not the paradox! The paradox is why the alfalfa grows less well in later years despite fixing nitrogen.
  3. C
    Certain types of plants, Remaining source text redacted.
    Why choice C matches the stem
    This answer provides something about alfalfa that explains why it grows less well when replanted over and over again in the same field. It releases toxins that accumulate in the soil, thus preventing it from growing well after a few years.
  4. D
    Alfalfa increases nitrogen in Remaining source text redacted.
    Why choice D is not credited
    First, this answer doesn’t establish that alfalfa without the bacteria decreases the amount of nitrogen in the soil - it may leave it untouched. Second, there’s also no indication that the bacteria isn’t present in essentially all soil, or at least all soil that alfalfa is planted in. And even then, it still wouldn’t explain the decline in how well it grows, as presumably the bacteria doesn’t just randomly disappear from the soil after a few years of living with alfalfa.
  5. E
    Alfalfa is very sensitive Remaining source text redacted.
    Why choice E is not credited
    There’s so much out of scope with this answer choice… Let’s just leave it at there’s no indication that black walnut trees (or any other source of juglone) is present anywhere near these fields. It also doesn’t say that “sensitive” means that the alfalfa would grow less well - maybe it’s “sensitive” in that even little bit supercharges growth!

What this tests

Question analytics

Based on historical answer selection rates for this question.

Answer choice distribution

  1. A 10%
  2. B 7%
  3. C Credited 72%
  4. D 10%
  5. E -100%

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