LSAT Question Stem

The argument depends on assuming which one of the following? 

Logical Reasoning Question Type

This is a Necessary Assumption question. 

Correct Answer

The correct answer to this question is A. 

LSAT Question Complete Explanation

To begin, let's analyze the structure of the argument in the passage. The passage discusses brown dwarfs and red dwarfs, which are similar in appearance but have different properties. Brown dwarfs are too cool to burn hydrogen, while red dwarfs are just hot enough to do so. The passage then states that all stars, when first formed, contain substantial amounts of lithium. However, all stars except the coolest brown dwarfs are hot enough to destroy lithium completely by converting it to helium. The conclusion of the argument is that any star found that contains no lithium is not one of these coolest brown dwarfs.

Now, let's break down the argument structure:

1. Premise: All stars when first formed contain substantial amounts of lithium.

2. Premise: All stars except the coolest brown dwarfs can destroy lithium completely by converting it to helium.

3. Conclusion: Any star found that contains no lithium is not one of these coolest brown dwarfs.

An "Evaluate" question for this argument could be: "Have the coolest brown dwarfs always been too cool to destroy lithium?"

The question type for this problem is Necessary Assumption, and we are asked to identify the assumption on which the argument depends.

Answer Choice Explanations:

a) This answer choice is correct. The argument assumes that the coolest brown dwarfs have never been hot enough to destroy lithium. If this assumption is not true, then the conclusion of the argument would be invalid.

b) This answer choice is incorrect because it discusses most stars that are too cool to burn hydrogen, which is not directly relevant to the argument. The argument focuses on the coolest brown dwarfs, not most stars that are too cool to burn hydrogen.

c) This answer choice is irrelevant and incorrect, as the argument does not make any claims or assumptions about the destruction of helium. The focus of the argument is on the destruction of lithium and its relation to the coolest brown dwarfs.

d) This answer choice is incorrect because the argument does not need to assume that most stars contain roughly the same percentage of lithium when first formed. The argument only states that stars contain substantial amounts of lithium, and it does not rely on the assumption that these amounts are roughly equal.

e) This answer choice is incorrect because the comparison between brown dwarfs and red dwarfs is not a necessary assumption for the argument. The argument's conclusion is about the coolest brown dwarfs and their relationship to lithium, not their similarity in appearance to red dwarfs.