LSAT Explanation PT 23, S3, Q18: When a stone is trimmed by

LSAT Question Stem

Which one of the following, if true, most seriously weakens the argument? 

Logical Reasoning Question Type

This is a Weaken question. 

Correct Answer

The correct answer to this question is B. 

LSAT Question Complete Explanation

The question type for this problem is Weaken, and our goal is to find the answer choice that most seriously weakens the argument presented in the passage.

First, let's analyze the argument in the passage. The passage tells us that when a stone is trimmed and exposed to the elements, rock varnish accumulates on its surface. Organic matter found beneath the varnish on stones of an Andean monument was determined to be over 1,000 years old. The passage then concludes that the monument must have been built long before the arrival of Europeans in the Americas in 1492.

In this argument, the premise is that the organic matter found beneath the rock varnish is over 1,000 years old. The conclusion is that the monument was built long before 1492. The argument assumes that the age of the organic matter accurately reflects the age of the monument.

Our "Evaluate" question for this argument would be: "Is there any other explanation for the age of the organic matter found beneath the rock varnish that would not support the conclusion that the monument was built long before 1492?"

Now, let's examine the answer choices:

a) Rock varnish itself contains some organic matter.

This answer choice is irrelevant to the argument because the passage concerns organic matter under the varnish, not inside it. This choice does not provide any alternative explanation for the age of the organic matter, so it does not weaken the argument.

b) The reuse of ancient trimmed stones was common in the Andes both before and after 1492.

This answer choice is correct because it provides an alternative explanation for the age of the organic matter found beneath the rock varnish. If the monument was built using older, reused stones, then the age of the organic matter would not accurately reflect the age of the monument, thus weakening the argument.

c) The Andean monument bears a striking resemblance to monuments found in ancient sites in western Asia.

The appearance of the monument and its resemblance to other monuments is not relevant to the argument about the age of the organic matter and the monument's construction date. This choice does not weaken the argument.

d) The earliest written reference to the Andean monument dates from 1778.

This answer choice does not provide any information about when the monument was built or whether the organic material found within is an accurate indication of the age of the monument. It does not weaken the argument.

e) Rock varnish forms very slowly, if at all, on trimmed stones that are stored in a dry, sheltered place.

This answer choice is vague and does not provide any specific information that weakens the argument. We don't know what "very slowly" means in this context, and it does not offer an alternative explanation for the age of the organic matter.

In conclusion, the correct answer choice is (b), as it provides an alternative explanation for the age of the organic matter found beneath the rock varnish, which weakens the argument that the monument was built long before 1492.