During the Carboniferous period, Virginia was equatorial and the warm seas supported abundant life and the deposition of lime sediments. The limestones produced from these sediments contain fossils of bryozoans and relatives of sea urchins and sand dollars. On land, coastal swamps formed along what is now the Appalachian Plateau. Tree ferns, horsetail rushes, and other plants grew in abundance in these swampy lowlands. This warm, tropical environment was the original source of what natural resource mined throughout this area during the past century? A) coal B) gypsum C) kaolin D) methane