It can be inferred from the passage that the supporters of the "layered-mantle" theory believe which of the following? A. The volcanic rocks on oceanic islands are composed of material derived from the lower part of the mantle. B. The materials of which volcanic rocks on oceanic islands and midocean ridges are composed are typical of the layers from which they are thought to originate. C. The differences in composition between volcanic rocks on oceanic islands and the midocean ridges are a result of different concentrations of incompatible elements.