In the passage, the author is primarily concerned with doing which one of the following? A. describing three theories and explaining why the latest of these appears to be the best of the three B. attacking the assumptions inherent in theories that until the 1980s had been largely accepted by geologists C. outlining the inadequacies of three different explanations of the same phenomenon D. providing concrete examples in support of the more general assertion that theories must often be revised in light of new evidence E. citing evidence that appears to confirm the skepticism of geologists regarding a view held prior to the 1980s