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- Mon Dec 21, 2015 12:00 am
#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
Phosphorus increase Plankton growth Bacterial consumption
Oxygen depletion 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 B C;
therefore, A 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.
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
Phosphorus increase Plankton growth Bacterial consumption Oxygen depletion Few fish surviveAnswering 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
B C;therefore, A
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.