This was before the steam locomotive, and canal building was at its height. The companies

This was before the steam locomotive, and canal building was at its height. The companies building the canals to transport coal needed surveyors to help them find the coal deposits worth mining as well as to determine the best courses for the canals. This job gave Smith an opportunity to study the fresh rock outcrops created by the newly dug canal. He later worked on similar jobs across the length and breadth of England, all the while studying the newly revealed strata and collecting all the fossils he could find. Smith used mail coaches to travel as much as 10,000 miles per year. In 1815 he published the first modern geological map, "A Map of the Strata of England and Wales with a Part of Scotland," a map so meticulously researched that it can still be used today.

Which of the sentences below best expresses the essential information in the highlighted sentence in paragraph 3？ 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 some day 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.

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