Samples from the floor of a rock shelter in Pennsylvania were dated by analyzing the carbon they contained. The dates...

Victoria on February 5, 2021

Hi @Remi,

Happy to help!

The question stem asks us to identify the answer choice which argues most strongly against the suggestion of the skeptics.

What do the skeptics suggest?

The passage tells us that samples from the floor of a rock shelter were dated and associated with human activities based on carbon content and depth.

The oldest and deepest sample was dated ~20,000 years before the present.

The skeptics suggest that the samples could have been contaminated by "old carbon" which was carried there by percolating groundwater.

Why? Because the skeptics believe that the suggested age of the sample does not correspond with the accepted date of human migration to North America.

We are looking for the answer choice which conflicts with the skeptics' suggestion that the samples were contaminated by carbon which was carried there by percolating groundwater.

Answer choice (B) is incorrect because we do not know if the skeptics are scientists.

Answer choice (C) is incorrect because the skeptics' suggestion does not require that people were using coal for fuel. They suggest that the carbon was from coal deposits.

To eliminate answer choices (D) and (E), it is important to remember that we are focused on the skeptics' specific suggestion, not their general skepticism. In other words, we are focused on the suggestion that the sample was contaminated by percolating groundwater NOT the view that the date is inaccurate.

Therefore, answer choices (D) and (E) are incorrect because they do not address the skeptics' suggestion that the sample was contaminated by groundwater. They merely support the idea that the date of the samples is inaccurate.

This leaves us with answer choice (A). This is the correct answer because it refutes the skeptics' suggestion that the deepest sample was contaminated via percolating groundwater. If no likely mechanism of contamination involving percolating groundwater could affect the deepest level without also affecting the uppermost level, then the top sample would also be contaminated. However, we are not provided with any information to suggest that this is true.

Hope this helps! Please let us know if you have any further questions.

MasonDees on October 19, 2022

If the passage doesn't state that the top samples aren't contaminated, how does A weaken the skeptics' position? Couldn't they say the top layer also is contaminated?

Emil-Kunkin on October 24, 2022

Hi, that is indeed a possibility. Maybe it is contaminated near the top. But we dont need to kill the argument, only to plausibility weaken it.

MasonDees on October 25, 2022

Okay thanks