Unplugged Practice Test: Logical Reasoning Set 2, Question 15 (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
Geologist: A survey of the flooded chamber at Ardenne Cave recorded a surge in dissolved iron that was gone again within a day. Hydrothermal seeps are known to produce surges of that kind elsewhere, and the chamber's walls carry mineral crusts left by seepage sometime in the past. Even so, it would be wrong to infer that a seep produced the surge at Ardenne. No active seep has been located in the chamber, and the chemistry of groundwater is known to vary in cycles of its own accord.
15. Which one of the following, if true, most undermines the geologist's reasoning?
- (A)The silt suspended throughout the chamber would have kept the survey from spotting an active seep even if one were operating there.
- (B)Most surges of dissolved iron in the region's surface streams come from runoff off abandoned mine tailings.
- (C)A record kept 40 years ago describes a similarly brief rise in dissolved iron in the same chamber.
- (D)Dissolved iron stands at higher levels in the cave's water than in the region's surface streams over the long run.
- (E)Water in a chamber where a hydrothermal seep has gone quiet holds measurably elevated iron for years afterward.
AWhy (A) is correct
The geologist accepts that hydrothermal seeps produce short iron surges elsewhere and that Ardenne's walls even carry old seepage crusts, then resists pinning the new surge on a seep because no active seep turned up in the chamber and groundwater chemistry cycles on its own anyway. That claim that no active seep has been located only counts against a seep if the survey would have caught one operating there. (A) knocks that out: silt suspended throughout the chamber would have kept the survey from spotting a seep, so it could have missed a real one entirely, and the fact that none was found stops proving anything.
Why the other answers are wrong
BWhy (B) is wrong
(B) points to surface-stream iron surges tied to mine tailings, a source that has nothing to do with a flooded cave chamber, so it doesn't touch the geologist's reasoning.
CWhy (C) is wrong
(C) reports an earlier similar rise in the same chamber, which is just what the geologist's own cyclical-variation explanation predicts.
DWhy (D) is wrong
(D) compares long-run iron levels between the cave and surface streams, which says nothing about what caused this one-day surge.
EWhy (E) is wrong
(E) shows that a seep gone quiet leaves iron levels high for years, but the Ardenne surge vanished in a day, a mismatch that fits the geologist's skepticism rather than weakening it.
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.
