Logical reasoning PrepTest 109 · Section 3 · Question 18

Incorrect. The conclusion here is that something is "even more healthful" than another not that it's purely healthy.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Flawed Parallel Reasoning

Stimulus Summary:
Photosynthesis benefits from higher CO2 levels, and those levels are increased by industrial activities, so these industrial activities are purely good.

Answer Anticipation:
There's a key phrase in the conclusion of this argument that we can focus to highlight the likely flaw, even if we hadn't seen the premises—"purely beneficial." When an argument reaches such a strong ("purely") conclusion it needs equally strong premises to back it up. Here to prove that these activities are purely beneficial it would need to be shown that there are benefits to them but no downsides.

While the premises do establish that there's at least one benefit to them (higher CO2 levels leading to more photosynthesis/life—sustaining proteins) it doesn't discuss a lack of downsides. As such it hasn't really completed the full pro/con list that would be needed to back up the conclusion (and that con list would need to be established as being empty).

Since we've identified a flaw and can tie it to specific language in the conclusion we should start our analysis of each answer with the conclusion to see if there's similar language—something that suggests a given phenomenon has only upsides and no downsides. That's inherently related to the flaw/assumption here so we can get rid of any answer choice that doesn't match up.

Answer Explanation:
This answer has a conclusion stating that something has "a lot of good" but "no harm matching up with the "purely beneficial" language in the conclusion of the stimulus. If the premises bring up a positive of something without eliminating the possibility of a downside then this is our answer. And that's what it does! It shows that exercise can prevent diseases but it never establishes that there are no downsides (anyone who's injured themselves while running knows that they exist). This answer commits the same flaw as the stimulus so this is the correct answer.

Key Takeaway:
Identifying key language or concepts related to a flaw can be an easy way to identify it in an argument and an easy way to eliminate answers that aren't parallel in a Flawed Parallel Reasoning question. By noting that the word "purely" was instrumental to the flawed logic in the stimulus we were able to make some quick eliminations in answers choices that didn't have a similar concept.

Incorrect. The conclusion here makes a recommendation ("ought") which the stimulus doesn't do. It also never says that the recommended action—fasting—has no downsides; just that it should be done because it can help in a certain way.

Incorrect. The language in this conclusion is equally as strong as the stimulus, but in a different way. The stimulus concludes that a certain action is purely beneficial; the conclusion here recommends ("ought" another way it's not parallel) to completely abandon one thing in favor of another without establishing that that action has no downsides. Different flaw.

Incorrect. The conclusion here is that something is the "best" policy not that it lacks downsides. Something can be the best in spite of having some negative effects as long as they're balanced by the positives to lead to better outcomes than other options.