Unplugged Practice Test: Logical Reasoning Set 3, Question 2 (Weaken)
Taking the test? Finish it first. This page shows the answer.
- Weaken questions ask which new fact most undermines the argument.
- Find the gap the argument leaves open, then look for the choice that widens it.
- Every question on this test is original, written to the same logical skeletons as real questions, and Steve checked every one himself before release.
The question
The speed at which a glass bottle cooled just after molding can be worked out from the balance between two stress bands that appear under polarized light. Such readings, taken from bottles the Renfro dairy has washed and refilled for decades, put the cooling speed at the neck close to the cooling speed at the base. On bottles shaped by hand press, the neck cools far more slowly than the base does. So the Renfro bottles were probably formed on an automatic machine.
2. Which one of the following, if true, most seriously weakens the argument?
- (A)Bottles formed on automatic machines cool slightly more slowly at the neck than at the base.
- (B)Sand fine enough for bottle glass was cheaper near the Renfro dairy 40 years ago than it is now.
- (C)Among bottles formed on automatic machines, small ones cool more evenly than large ones do.
- (D)Washing and refilling add stress bands that put a bottle's original balance beyond recovery.
- (E)Automatic machines shape bottles faster and melt more glass per hour than hand presses do.
DWhy (D) is correct
The argument reads the Renfro bottles' stress bands, a near-equal cooling speed at neck and base, as evidence they were shaped by machine rather than hand press, since hand-pressed bottles cool much more slowly at the neck. That inference only holds if the stress bands measured today still reflect the cooling that happened when the bottle was first molded. (D) undercuts it: if decades of washing and refilling add their own stress bands that make the bottle's original balance impossible to recover, the near-equal reading may reflect handling, not how the bottle first cooled, so it says nothing about which method shaped it.
Why the other answers are wrong
AWhy (A) is wrong
(A) is a small gap consistent with the Renfro reading, so it supports rather than weakens the machine conclusion.
BWhy (B) is wrong
(B) says nothing about cooling speed or forming method.
CWhy (C) is wrong
(C) doesn't address whether the Renfro readings still reflect original cooling at all.
EWhy (E) is wrong
(E) says nothing about which stress-band pattern a bottle ends up with.
Common questions
What type of question is this?
Weaken. Weaken questions ask which new fact most undermines the argument.
How do I get better at Weaken questions?
Find the gap the argument leaves open, then look for the choice that widens it. The free LSAT cheat sheet lists the wrong-answer patterns to watch for on every type.
