Many Pacific Island Nations Consist of a Collection of Atolls

Many Pacific Island nations consist of a collection of atolls.Some of these nations have appealed to the United Nations claiming that rising sea level is causing coastal flooding.Another explanation for coastal flooding on these islands other than rising sea level is

A) Atolls are ringed by coral reefs and these reefs are dying.
B) Atolls are susceptible to more beach erosion than other types of islands.
C) Atolls are former volcanoes that are slowly sinking (subsiding) .
D) Atolls are composed of limestone that is slowly dissolving.