Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information. A. The discovery of regional differences in the sequences of rocks led geologists to believe that rock types could someday become reliable time markers. B. Careful analysis of strata revealed that rocks cannot establish geological time because the pattern of rock layers varies from place to place. C. Smith's catalogs of rock strata indicated that the sequences of rocks are different from place to place and from region to region. D. Because people did not catalog regional differences in sequences of rocks, it was believed that rocks could never be reliable time markers.