Logical reasoning PrepTest 141 · Section 4 · Question 15

Correct. Argument or Facts:
Facts

Question Type:
Paradox

Stimulus Summary:
Dragonfly larvae need water and are preyed upon by several species including crayfish. However, their populations are healthier in areas with more crayfish.

Answer Anticipation:
The crayfish do pose a risk to the dragonflies—they're a known predator. However, they must do something else that helps at least as much as the predation hurts in order to explain why the populations are healthier in areas with a known predator. Maybe the crayfish also eat those other dragonfly predators, or they have some effect on the habitat that helps the dragonflies thrive.

Answer Explanation:
The stimulus states that dragonfly larvae need water to survive ("can survive only in the water"). If the crayfish maintain water in areas that otherwise go dry, then that would explain how the dragonfly populations are more likely to stay healthy in these areas with crayfish—they have places to breed that are missing from other areas.

Key Takeaway:
Unexpected outcomes—like prey thriving in areas with a predator—usually derive from some unknown piece of information. This stimulus follows a similar pattern of missing information—an incomplete list of pros/cons. Here, the presence of a predator is a con suggesting populations of prey would struggle to survive. Since they didn't, there must be a pro—something the predators bring that helps survival—that is unmentioned.

Incorrect. This doesn't explain why areas with the crayfish are more likely to see a healthy dragonfly population, as areas without the crayfish would still lack a predator of the larvae.

Incorrect. This answer rules out a possible explanation of the paradox! If the crayfish don't eat any of the other predators, then that removes one potential way their presence could lead to a thriving dragonfly population.

Incorrect. This answer doesn't resolve the issue of why the dragonflies are thriving in areas with crayfish but not in others. The paradox is primarily about where the dragonflies are, not the crayfish.

Incorrect. This answer may suggest that the dragonflies aren't the primary food source for the crayfish, but even that doesn't explain why the dragonflies thrive in the presence of a predator. Even if the crayfish primarily eat something else, they are still eating the dragonfly larvae.