Atolls have the most distinctive features of all coral reef types. A. Because of its many marine animal colonies, the Indo-Pacific area contains the world's largest number of atolls, barrier reefs, and fringing reefs. B. In the years after Darwin's voyage, researchers discovered that Darwin was incorrect in believing that all fringing reefs and barrier reefs will eventually become atolls. C. The discovery of guyots suggested to researchers that atolls would never form in non-volcanic islands where fringing and barrier reefs are common. D. Darwin argued that atolls are formed when volcanic islands surrounded by fringing reefs begin to slowly sink, forming first a barrier reef and finally an atoll. E. Darwin's theory was confirmed by the discovery of volcanic rock underneath the limestone layers of an atoll in the Marshall Islands. F. The fringing reefs and barrier reefs that grow in the waters of continental margins and around non-volcanic islands will never become atolls.