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#35270
Complete Question Explanation

Must Be True—CR. The correct answer choice is (A)

This stimulus describes a fairly lengthy causal chain. The chain begins with agricultural runoff,
which causes increased phosphorous levels. The phosphorous, in turn, promotes the growth of
plankton, which is eaten by bacteria. This consumes oxygen and finally results in few fish being able
to survive. Graphically, this could be represented as:

Agricultural runoff :arrow: Phosphorus increase :arrow: Plankton growth :arrow: Bacterial consumption
:arrow: Oxygen depletion :arrow: Few fish survive

Answering this Must Be True question requires properly understanding the causal chain.

Answer choice (A): This is the correct answer choice. The correct answer simply connects
the initial cause (agricultural runoff) with a secondary effect (plankton growth) by omitting the
immediate effect (phosphorus increase). This is similar, but not identical, to an additive inference in
conditional reasoning, which is drawn through a common condition (causally, A :arrow: B :arrow: C;
therefore, A :arrow: C).

Answer choice (B): In general, it is an error to draw inferences about prior conditions based on
causal reasoning. We cannot know what other factors might have endangered fish previously, nor
can we know how much phosphorus is needed to prevent most fish from surviving (i.e., even half of
the current level of phosphorous might have been detrimental to fish). We cannot correctly infer this
answer.

Answer choice (C): Knowing that agricultural runoff creates food for bacteria is not sufficient to
infer that bacteria require agricultural runoff to survive. Furthermore, “no bacteria” is too strong to
match the language of this stimulus and cannot be properly concluded.

Answer choice (D): Phosphorus levels have doubled, but there is no evidence that the increase in
phosphorus is directly proportional to the quantity of agricultural runoff. Since we cannot eliminate
the possibility that the agricultural runoff has become either more or less potent at producing
phosphorus in the past few decades, this answer must be incorrect.

Answer choice (E): Answer choice (E) is refers “a body of water” in general. The stimulus is about
a vast ocean region with a large river nearby. What is true of this region will not necessarily apply
to a puddle. Furthermore, remember that there are two intermediate steps between phosphorus
accumulation and oxygen depletion: namely, plankton growth and bacterial consumption. If either
of these steps were absent in another body of water, we would have no reason to connect phosphorus
levels and oxygen levels.
 bli2016
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#34607
Hi, for this question, I narrowed the answer choices down to A and E and then chose A because E sounded too specific (the stimulus never mentioned proportionality). Is my reasoning here correct?
 Adam Tyson
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#35030
It is, bli! Nice job! The concept of proportionality is new information, not supported by the facts given in the stimulus, and so it must be rejected.

Good work!
 jessamynlockard
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#45922
My tutor said that an additional reason was that "inversely proportional" in E implies a specific proportion, whereas negatively correlated wouldn't. Could you shed some light on this? Thanks.
 James Finch
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#46278
Hi Jessamyn,

"Inversely proportional" would mean that as you add more of one substance, the amount of the other substance would decrease, and vice versa. This doesn't necessarily imply a specific proportion or even a strictly linear relationship, but if one goes up, the other must necessarily go down, regardless of the numbers involved.

With answer choice (E), the scope is overly broad; we don't whether the proportionality would work at extreme levels of phosphorus or oxygen, nor do we know if this holds true for all bodies of water, only for the one specific ecosystem that the stimulus describes. Looking at both the correct answer choice, (A), and (E), (A) deals with the specific situation in the stimulus, while (E) is incredibly broad.

Hope this helps!

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