We know from the stimulus that human artifacts have been found on the floor in a cave in Pennsylvania. Carbon dating was used on the samples and the ones close to the surface date to recent time periods. The deepest ones are very old, so the age correlates well with the depth. There are some skeptics who believe that the oldest samples are simply too old, as the dates suggest they were made before humans migrated to North America. The skeptics think that the age that the carbon dating is suggesting is so old because the samples have become contaminated with coal.
We're looking for a reason as to why the skeptics might not be correct. It's possible that humans migrated earlier to North America than was previously thought, or perhaps there is some reason that it is not plausible for the coal to have caused contamination in the artifacts. We need to look for an answer that weakens the skeptics' position.
(A) says, "No likely mechanism of contamination involving percolating groundwater would have affected the deeper samples from the site without affecting the uppermost sample."
(A) is great. If the groundwater had actually contaminated some of the artifacts, then (A) is saying that the top artifacts would have also been contaminated. The stimulus gives us information that the most recent artifacts date from recent times, so they weren't contaminated. Thus, (A) tells us that none of the artifacts were contaminated with percolating groundwater, thereby weakening the skeptics' position.
Does this make sense? Let us know if you have any more questions!
masonnavarreteSeptember 25, 2019
Hi, I'm having a difficult time putting together how we know that the top artifacts are not contaminated. I understand that there is a progression and that the newest and most recent ones are on top, but can't the groundwater continue to contaminate the artifacts more than just during the oldest period?
SageApril 18, 2020
I agree with the latest comment. I'm not finding any part of the stimulus where it suggests that the "uppermost one" is NOT contaminated. Is this something that we are supposed to infer from the stimulus?
