Samples from the floor of a rock shelter in Pennsylvania were dated

There are literally many layers to this passage, so let's take the time to break it all down.

Skeptics suggest that samples associated with human activities could have been contaminated by dissolved old carbon carried by percolating groundwater from nearby coal deposits.

Here's how they arrived at this suggestion (i.e., conclusion):

• Samples from the rock shelter floor were carbon-dated.
• For samples associated with human activities, the assigned dates formed a consistent series.
• This series starts with the present day and goes back in time. The deeper the sample, the earlier the date of that sample.
• The oldest, deepest sample is dated at 19,650 years before present day (+/- 2,400 years).
• Skeptics believe that this date is too early to accept, because it doesn't match the accepted date of human migration into North America.
• Skeptics therefore suggest that the samples could have been contaminated -- specifically via percolating groundwater, which theoretically dissolved old carbon from nearby coal deposits, then carried that carbon into the rock shelter floor.

We're looking for the choice that most weakens the skeptics' suggestion that these samples have been contaminated by old carbon in percolating groundwater. We can't really challenge the accepted date of human migration to North America. And the claim that the samples were contaminated is really the heart of the skeptics' suggestion, so the potential means of contamination is especially open to question. Let's work our way through the answer choices.


Hmmm, not bad! The skeptics are reacting to the oldest and deepest sample. Since they don't agree with the date assigned to that sample, they're suggesting that the samples as a whole could have been contaminated by percolating groundwater.

But according to the passage, the top-layer sample dates to the present day. This dating of the top layer isn't called into question by anyone, not even the skeptics. So if (A) is true, then there's no way the samples could have been contaminated by percolating groundwater. (A) says that if this type of contamination had happened, then old carbon would have contaminated the top layer as well, and top-layer samples would NOT have been dated with the present day.

Consequently, if (A) is true, then it's very unlikely that the mechanism of contamination named by skeptics (percolating groundwater) was the cause of this unusually early date in the deepest sample. (A) isn't a watertight argument killer, but it knocks back the skeptics' reasoning in a very direct way. So let's keep this choice around.


Huh? What do the general opinions of scientists have to do with this particular series, this particular sample, this particular date, and these particular skeptics? Choice (B) doesn't come close to addressing the conclusion that we're trying to weaken, so eliminate it.


We don't care whether coal was ever being used for fuel. Unless there's some concrete connection between the uses of coal and the ways that coal could have contaminated these rock samples, then what people have done with coal is irrelevant. Eliminate (C).


Choice (D) is one of those options that sounds good but really says nothing at all. The passage already tells us that the order of dates in the series was consistent. This choice repeats the same fact in different wording. If (D) is true, we haven't learned anything new, and aren't closer to understanding why the deepest, earliest sample seems to be dated with a year that's too far in the past. So we'll eliminate (D), too.


Like choice (B), this choice points us to information that's way outside the scope of this argument and has very little to do with the specific conclusion we've been asked to weaken. If anything, this information would make us slightly more likely to believe the skeptics, because it tells us that this weird date truly is an outlier. So we'll get rid of (E).

(A) is the only choice that directly addresses the logic presented by the skeptics, then offers information to cast doubt on that logic. None of the other choices come close, so even if we're getting to (A) solely through elimination, it's the best choice available.

I hope this helps! (...specifically, I hope this helps you rock the GMAT? Haha. Yeah, I know: my puns are getting worse.)