Logical reasoning PrepTest 158 · Section 3 · Question 19

This answer choice provides the inverse of our anticipated sufficient premise. Establishing that acidic ponds are not beneficial for turtles would justify this conclusion, as Wallakim is an acidic pond. But establishing that non-acidic ponds are beneficial does not tell us anything about Wallakim, and therefore does not support this conclusion.

This answer choice establishes a link between a pond's acidity and its turtle population, so it at least addresses the gap in the argument. However, it doesn't establish that acidity is bad or negatively impacts the turtle population. Even if (B) is true, it could be the case that an acidic pond is actually beneficial to turtle populations. So (B) doesn't ensure the validity of this argument's conclusion.

Even if the waters at Sosachi a more beneficial than the waters at Wallakim, that doesn't establish that the waters at Wallakim are not beneficial to turtles. Both Sosachi and Wallakim could have beneficial conditions for turtles, with Sosachi featuring slightly better conditions.

This answer choice describes a hypothetical about Wallakim Pond — if it was not acidic, it would be beneficial to turtles. One implication of this answer choice is that it's beneficial to turtles if a pond isn't acidic. That's the same conditional relationship that answer choice (A) brings up, so this answer choice is wrong for the same reasons as (A). We know Wallakim is acidic, and establishing that non-acidic ponds are beneficial doesn't tell us anything about Wallakim.

Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Strengthen with a Sufficient Premise

Stimulus Summary:
If turtles can thrive in a pond, then the pond's conditions must be beneficial to turtles. Wallakim Pond is acidic, so turtles aren't thriving there.

Strategy Overview:

1. Argument or facts? Always argument, so identify premises and conclusions of argument
2. Anticipate why the premises are not enough to show that the conclusion is true
3. Use anticipations to select the answer choice that:
   1. Guarantees the truth of the conclusion

Answer Anticipation:
On Strengthen with a Sufficient Premise questions, we need to find the answer choice that would make the conclusion valid if added as another premise. Before we can figure that out, we'll need to identify the premises and conclusion. Fortunately, they're not trying to hide this conclusion. The word "Thus" indicates that the last sentence is a conclusion. The preceding two sentences are premises. (For the purposes of this question, we can ignore the information about Sosachi Pond, since the conclusion is just about Wallakim Pond.)

About half of these questions include conditional statements, so it helps to scan the premises and conclusion for sufficient and necessary keywords. This first premise features conditional keywords ("for," referring to a sufficient condition; "must," referring to a necessary condition), so we can diagram out this argument:

1. Thriving Turtle Pop. → Beneficial Conditions
2. Wallakim: Acidic

Conclusion: Wallakim: NOT Thriving Turtle Pop.

The conclusion attempts to prove that Wallakim does not have a thriving turtle population. To make the first premise consistent with that, we’ll need to take the contrapositive:

1. Contrapositive: NOT Benefificial Conditions → NOT Thriving Turtle Pop.
2. Wallakim: Acidic

Conclusion: Wallakim: NOT Thriving Turtle Pop.

To justify this conclusion, we'll need to establish that Wallakim does not have beneficial conditions for turtles. The second premise establishes that Wallakim is acidic, so one way to prove that Wallakim does not have good conditions for turtles is to state, "If a pond is acidic, then it doesn't have beneficial conditions for turtles." Or its contrapositive, "If a pond has beneficial conditions for turtles, then it cannot be acidic." 

Answer Choice Explanation:
This is our anticipated answer, and it will ensure the conclusion's validity. We can diagram this as:

Beneficial Conditions → NOT Acidic

If we take its contrapositive and add it to our argument, we can see that we have drawn a valid conclusion:

(E) Contrapositive: Acidic → NOT Benefificial Conditions

1. Contrapositive: NOT Benefificial Conditions → NOT Thriving Turtle Pop.
2.  Wallakim: Acidic

Conclusion: Wallakim: NOT Thriving Turtle Pop.

Key Takeaway:
Whether you are diagramming the premises and conclusion or simply anticipating a logical connection between the two, the correct answer to a Strengthen with a Sufficient Premise question is often a conditional statement. Prudent LSATers write out their anticipated answers as conditional statements, to eliminate negate-but-didn't-reverse and reversed-but-didn't-negate answer choices.