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

Question

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 came. The oldest and deepest sample was dated at 19,650 years before the present, plus or minus 2,400 years. Skeptic, viewing that date as too early and inconsistent with the accepted date of human migration into North America, suggested that the samples could have been contaminated by dissolved old carbon carried by percolating groundwater from nearby coal deposits.

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

A. No likely mechanism of contamination involving percolating groundwater would have affected the deeper samples from the site without affecting the uppermost sample.

B. Not every application of the carbon-dating procedure has led to results that have been generally acceptable to scientists.

C. There is no evidence that people were using coal for fuel at any time when the deepest layer might have been laid down.

D. No sample in the series, when retested by the carbon-dating procedure, was assigned an earlier date than that assigned to a sample from a layer above it.

E. No North American site besides the one in Pennsylvania has ever yielded a sample to which the carbon-dating procedure assigned a date that was comparably ancient.

Source: LSAT

GMATNinja · Accepted Answer

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.)

sobby · Answer

A and C seems as weakener ..but we are not concerned about the usage of coal ..coal can be there without being used by any civilization ...that eliminates C 
A is best among lot ..that says because of percolating ground water ,all the layer would have been equally effected .....not only last layer ..

unraveled · Answer

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 came. The oldest and deepest sample was dated at 19,650 years before the present, plus or minus 2,400 years. Skeptic, viewing that date as too early and inconsistent with the accepted date of human migration into North America, suggested that the samples could have been contaminated by dissolved old carbon carried by percolating groundwater from nearby coal deposits.

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

A. No likely mechanism of contamination involving percolating groundwater would have affected the deeper samples from the site without affecting the uppermost sample. - CORRECT. At first look, it looks to support but it actually does not.

B.  Not every application of the carbon-dating procedure  has led to results that have been generally acceptable to scientists. - WRONG. True but does not affect the passage.

C. There is  no evidence that people were using coal for fuel at any time  when the deepest layer might have been laid down. - WRONG. Touches slightly the core aspect but with an approach that we are not concerned about. Since contamination is not dealt with, this is wrong.

D. No sample in the series,  when retested by the carbon-dating procedure , was assigned an earlier date than that assigned to a sample from a layer above it. - WRONG.

E.  No North American site besides the one in Pennsylvania  has ever yielded a sample to which the carbon-dating procedure assigned a date that was comparably ancient. - WRONG.

POE helps in this one.

Answer A.

VerbalBot · Answer

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

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

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615 to 715 in 15 Days (Ria's Strategies)

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