LSAT 91 - Section 2 - Question 19

This is a Sufficient Assumption question.

The argument starts with a conditional: a thriving population of turtles in a pond requires beneficial conditions at the pond.

thriving → beneficial

Wallakim Pond, we’re told, has acidic water.

acidic_w

We’re also told that Sosachi Pond doesn’t but the conclusion doesn’t care about Sosachi and so we shouldn’t either.

Finally, the conclusion says that the population of turtles at Wallakim Pond must not be thriving.

/thriving_w

Let’s put this all together.

thriving → beneficial

acidic_w

_________________

/thriving_w

Looking at the conclusion, you can see that the argument is trying to contrapose on the conditional. It’s trying to fail the “beneficial” condition. If it’s successful in doing that, then we can conclude “/thriving.” But the problem is that the only other premise doesn’t hook up to “/beneficial.” We don’t know what “acidic” means for turtles. Is that beneficial for them or not? If we’re able to establish that “/acidic” is a necessary condition, then this argument becomes valid:

thriving → beneficial → /acidic

acidic_w

_________________

/thriving_w

This is what Correct Answer Choice (E) gives us. It says that the conditions of a pond are beneficial only if the water is not acidic. That’s exactly what we’re looking for: beneficial → /acidic.

Answer Choice (A) says that if the water is not acidic, then the conditions are beneficial. /acidic → beneficial. That’s the sufficiency-necessity confused version of (E).

Answer Choice (B) says that the acidity of water is the most important factor that determines whether the population of turtles will be thriving. But that doesn’t tell us whether acid is good or bad. It just says it’s powerful. In which direction? Even if (B) said that it’s in the bad direction, it would merely strengthen the argument, which would still fall short of the SA requirement.

This is what we in effect get in Answer Choice (C). It says that the conditions at Sosachi are more beneficial than the conditions at Wallakim. We have to assume that all other conditions are held equal between Sosachi and Wallakim. On that assumption, we can infer that the difference is caused by the difference in their waters’ acidity. But even then, it just means that acidity is relatively less beneficial.

Answer Choice (D) says that Wallakim would have a thriving population if the water were not acidic. That translates to /acidic → thriving. But that doesn’t fit what we’re looking for.

That means it’s the acidity in the water that’s causing the population to not be thriving.