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. Skeptics, viewing that date as to 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 of the following considerations, if true, argues most strongly against the suggestion of the skeptics? ['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.', 'Not every application of the carbon-dating procedure has led to results that have been generally acceptable to scientists.', 'No likely mechanism of contamination involving percolating groundwater would have affected the deeper samples from the site without affecting the uppermost sample.', 'There is no evidence that people were using coal for fuel at any time when the deepest layer might have been laid down.']