LSAT Explanation PT 31, S2, Q2: For the last three years, entomologists

LSAT Question Stem

Which one of the following is an assumption on which the argument relies? 

Logical Reasoning Question Type

This is a Necessary Assumption question. 

Correct Answer

The correct answer to this question is D. 

LSAT Question Complete Explanation

Let's start by analyzing the argument in the passage. The passage's structure is as follows:

Premise 1: Entomologists have been searching for a parasite to control a whitefly that has become a serious crop pest, believing it to be a variety of sweet-potato whitefly.

Premise 2: Genetic research shows the pest is actually a distinct species, the silverleaf whitefly.

Conclusion: The search for a parasite has so far been wasted effort.

The question type is Necessary Assumption, which means we need to identify an assumption that the argument relies on for the conclusion to be valid.

Now, let's discuss each answer choice:

a) All varieties of the sweet-potato whitefly are serious crop pests.

This answer choice is not a necessary assumption because it doesn't affect the conclusion about the wasted effort. The problem is not whether all sweet-potato whiteflies are pests, but rather that the entomologists were searching for a parasite for the wrong species.

b) If a crop pest has a parasite, that parasite can always be used to control that pest.

This answer choice is not necessary either, as it merely strengthens the premise that a parasite can help control the pest. The issue remains that the entomologists were searching for a parasite for the wrong species.

c) The chances of successfully identifying a useful parasite of the new pest have increased since the proper identification of the pest.

This answer choice actually undermines the conclusion, suggesting that the search for a parasite might have been useful, even if it was in the wrong direction. This doesn't help us identify the necessary assumption for the conclusion.

d) No parasite of the sweet-potato whitefly is also a parasite of the silverleaf whitefly.

This is the correct answer. The argument assumes that the parasites of the sweet-potato whitefly cannot be used to control the silverleaf whitefly. If this assumption is not true, then the conclusion that the search has been wasted effort would not hold.

e) In the last three years, the entomologists found no parasites of the sweet-potato whitefly.

This answer choice is not a necessary assumption because it doesn't directly address the wasted effort. Even if the entomologists found parasites for the sweet-potato whitefly, it doesn't tell us whether those parasites would be effective against the silverleaf whitefly. If they were effective, then the search would not have been wasted.

An "Evaluate" question for this argument could be: "Do any parasites of the sweet-potato whitefly also affect the silverleaf whitefly?" The answer to this question would help determine whether the search for a parasite has indeed been wasted effort or not.