Logical reasoning PrepTest 142 · Section 2 · Question 10

Correct. Argument or Facts:
Facts

Question Type:
Must Be True

Stimulus Summary:
Agricultural runoff caused phosphorus levels in an ocean region to double, which caused plankton to grow, which caused more dead plankton on the ocean floor, which caused bacteria to consume oxygen, which caused most fish to die off.

Answer Anticipation:
This Must Be True question creates quite a causal chain! All that causal language and overlapping terms should be diagrammed out so the connection can be tracked.

Note that these are causal connections ("due to," "stimulates," etc . . . ), not conditional ones, so we can't take the contrapositive of the chain—though trap answers might do just that! And, while many Must Be True questions use all of the premises in justifying the correct answer, enough don't that we should make sure we have the entire chain mapped out in case the correct answer uses only a part of it instead of just looking for the end points.

Answer Explanation:
Looking at our chain, that agricultural runoff increased phosphorus levels which stimulated the growth of plankton near the ocean surface. While it only uses about half of the stimulus, this causal connection is backed up by the chain established there, so it's the correct answer.

Key Takeaway:
Two takeaways. First, in Must Be True questions, answers that talk about what would have happened in a different situation are rarely correct, as they rely on too many assumptions. Second, answers that establish or rely on directly proportional or inversely proportional relationships are also rarely correct—they require quite a bit of evidence to establish.

Incorrect. There's no information about this region from before the causal chain kicked in, so this answer choice is suspect. It's also problematic in stating that "most" fish were able to survive in this region. While the argument establishes that the doubling of phosphorus levels has led to "few" fish being able to survive, it's possible that many but not most fish were able to survive ahead of the runoff (say, 49%), but now that number has dropped to single digits.

Incorrect. First, taking the cause away doesn't mean that an effect will disappear—it's possible the damage has been done and there's no going back. Second, this answer is very extreme in stating that no bacteria would be on the ocean floor—presumably, there was a lower level of plankton decaying to the ocean floor before, which could have sustained a certain amount of bacteria.

Incorrect. This answer choice is trying to make you think that a doubling of phosphorus levels requires a doubling of agricultural runoff, since that runoff is the cause of those levels doubling. However, there doesn't need to be a direct relationship between the amounts of those two—it's possible that to double phosphorus levels in the ocean, the agricultural runoff would need to triple, or even increase by only 10%. In assuming a directly proportional relationship this answer choice is wrong.

Incorrect. Similar to (D), the direct relationship established here is too specific based on the stimulus. While it is established that phosphorus levels going up here caused oxygen levels to go down, that relationship might not be inversely proportional.