Samples from the floor of a rock

A free, expert breakdown of this official LSAT Logical Reasoning question.

TopicsWeaken

Keep going in LSAT Lab

Save & drill this skill — build targeted practice sets from questions like this one
Video walkthroughs — watch every question solved step by step
81 official LSATs — as questions, timed sections & full-length tests

Free to start

Full official LSAT questions are available through LawHub. This page provides LSAT Lab's explanation, strategy, and review tools without republishing the full official question.

Stimulus

Samples from the floor of a rock shelter in Pennsylvania were dated by analyzing the carbon they contained. The dates assigned to samples associated with human activities formed a consistent series, beginning with the present and going back in time, a series that was correlated with the depth from which the samples could have been contaminated by dissolved “old carbon” carried by percolating groundwater from nearby coal deposits.

What this question is testing

Weaken

Your task

Find the choice that makes the argument's conclusion less likely to be true.

Common trap

Answers that look negative but attack a claim the argument never relied on.

Winning move

Find the assumption the argument depends on, then pick the choice that undermines it.

Reading along? Open the full official question in LawHub — we show a fragment here and keep the reasoning in our own words.

The question

17.

Which of the following considerations, if true, argues most strongly against the suggestion

Answer choices

Correct70% picked this
No likely mechanism of contamination involving percolating groundwater would have affected the deeper samples from the site without
Why this is right
This makes the skeptics explanation seem implausible by pointing out that any contamination from groundwater should have logically affected all samples, not just deeper ones. Since the uppermost samples don't have the same "contaminated" date as the lower samples, it doesn't look like the lower samples were contaminated.
Skill tested: Weaken · how this choice captures the argument's function is the move to repeat next time.
70%
Strengthens (if anything)2% picked this
Not every application of the carbon-dating procedure has led to results that have been generally
This is an incredibly weak idea, so very likely to be correct on Str/Wkn/Pdx. Additionally, saying that "at least one application of carbon-dating has NOT led to acceptable results" would help the skeptics argue that this suspicious carbon dated sample is wrong.
2%
No Impact11% picked this
There is no evidence that people were using coal for fuel at any time when the deepest layer
The skeptics' contamination theory doesn't depend on using coal for fuel, but rather on natural soil/groundwater interactions, so this choice doesn't effectively counteract the contamination claim.
11%
No Impact15% picked this
No sample in the series, when retested by the carbon-dating procedure, was assigned an earlier date than that assigned to a sample
Answer D highlights that no deeper sample is older than one above it, supporting the data's internal consistency but doing little to address whether groundwater could have made the bottom layer's date look earlier than it really was.
15%
Opposite Impact (strengthens)3% picked this
No North American site besides the one in Pennsylvania has ever yielded a sample to which the carbon-dating procedure assigned a
This would help the skeptics argue that this oldest dated sample must be screwed up in some way, since we've never seen a comparably ancient sample elsewhere in North America.
3%

Continue the review in LSAT Lab

Save this question, watch the video walkthrough, and drill similar questions in your LSAT Lab account.

Save and practice this skill

LSAT Lab

Turn this review into a targeted study plan.

Save this question, drill more like it, watch the video walkthrough, and track your progress in your LSAT Lab account.

Start practicing free