LSAT Explanation PT 19, S4, Q9: Measurements of the motion of the

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 of this problem is Weaken, which means we need to find an answer choice that weakens the conclusion of the argument.

First, let's break down the argument in the passage:

Premise 1: Measurements of Uranus' motion show it being pulled away from the Sun and inner planets.

Premise 2: Neptune and Pluto don't have enough mass to exert the force indicated by the measurements.

Conclusion: There must be at least one undiscovered planet in our solar system causing this force.

To help understand this argument, let's use a simple example. Imagine you're playing tug-of-war, and you notice your team is being pulled towards the other side. You see that the other team has two strong players, but they aren't strong enough to cause the pull you're experiencing. So, you conclude there must be another hidden player on the other team.

An "Evaluate" question for this argument could be: "Is there any other force or object that could be causing the pull on Uranus?"

Now, let's analyze the answer choices:

a) Pluto was not discovered until 1930.

This answer choice is irrelevant to the cause of the force being exerted on Uranus, so it doesn't weaken the argument.

b) There is a belt of comets beyond the orbit of Pluto with powerful gravitational pull.

This is the correct answer choice because it provides an alternate explanation for the force pulling Uranus away from the Sun and inner planets. If the belt of comets is causing the pull, then the conclusion that there must be an undiscovered planet is weakened.

c) Neither Neptune nor Pluto is as massive as Uranus.

This answer choice doesn't weaken the conclusion, as it doesn't address the cause of the force on Uranus. It simply restates that Neptune and Pluto are smaller than Uranus, which doesn't affect the argument.

d) The force the Sun exerts on Uranus is weaker than the force it exerts on the inner planets.

This answer choice is tempting, but it doesn't account for the force pulling Uranus away from the Sun and inner planets. The argument states that there is a force tugging on Uranus, so we need to find an alternate explanation for that force, which this answer choice doesn't provide.

e) Uranus' orbit is closer to Neptune's orbit than it is to Pluto's.

This answer choice doesn't weaken the conclusion because the proximity of Neptune and Pluto's orbits doesn't address the cause of the force pulling on Uranus. It simply provides information about their relative positions, which doesn't affect the argument.

In summary, the correct answer choice is (b), as it provides an alternate explanation for the force pulling on Uranus, weakening the conclusion that there must be an undiscovered planet in our solar system causing this force.