On the basis of the available evidence, Antarctica has generally been thought to have been covered by ice for at least the past 14 million years. Recently, however, three-million-year-old fossils of a kind previously found only in ocean-floor sediments were discovered under the ice sheet covering central Antarctica. About three million years ago, ttherefore, the Antarctic ice sheet must temporarily have melted. After all, either severe climatic warming or volcanic activity in Antarctica' s mountains could have melted the ice sheet, thus raising sea levels and submerging the continent. Which one of the following is the main conclusion of the argument? ['What caused Antarctica to be submerged under the sea was the melting of the ice sheet that had previously covered the continent.', 'It is not the case that ancient fossils of the kind recently found in Antarctica are found only in ocean-floor sediments.', 'The ice sheet covering Antarctica has not been continuously present throughout the past 14 million years.', 'Antarctica is no longer generally thought to have been covered by ice for the past 14 million years.']