Logical reasoning PrepTest 142 · Section 4 · Question 22

Incorrect. There's no causation in the stimulus.

Incorrect. This answer describes an absence of evidence flaw. Here, the author does state that the fish are "possible related," suggesting the evidence isn't definitive. However, the relevant information to the application of the conditional premise is whether the fish are closely related, and that is definitively established by the argument.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
Two species closely related → Teeth evolved only once
not Two species closely related
Therefore: not Teeth evolved only once

Answer Anticipation:
Noticing that conditional relationship about halfway through the stimulus is key in spotting the flaw here. After establishing the conditional relationship, the argument then establishes that the sufficient condition isn't met and concludes that the necessary condition isn't met, either. That's a classic conditional flaw—an illegal negation. The argument is assuming that a given sufficient condition is the only sufficient condition.

Answer Explanation:
While this answer is normally associated with an illegal reversal, it can also be used to describe an illegal negation. In treating the lack of a close relationship as sufficient to justify concluding the teeth evolved more than once, the argument is treating a close relationship as necessary for the teeth to have evolved only once. The reason that this answer can be used to describe either flaw is that they are the contrapositive of each other, and thus they're logically equivalent.

Key Takeaway:
Illegal reversal and illegal negations should be something you distinguish between while anticipating an answer. However, if the only conditional logic answer describes the other flaw, you should still select it (after confirming all other answers are wrong)—they're logically equivalent since the flaws are the contrapositive of each other:
Statement: A → B
Illegal negation: not A → not B
Illegal reversal: B → A
Those last two are the contrapositive of each other!

Incorrect. The argument doesn't have any premises that use equivalent probability language ("likely"), so this answer is incorrect.

Incorrect. The biologists are actually established to be wrong by the premise stating the fish aren't closely related, so the argument isn't appealing to their authority to justify its conclusion.