A) are a diverse group of cnidarians
B) are predominantly photosynthetic, multicellular algae
C) excrete external, carbonaceous skeletons
D) A and C only
E) B and C only
As far as I can recall, corals as cnidarians secrete (not excrete) calcareous skeletal components. Coral polyps harbor symbiotic, photosynthetic zooxanthellae (therefore B is technically false), as these photosynthesize they produce CO2 that retards calcium dissolution, enhancing the precipitation of the hard part of the coral. The coral polyps themselves are carnivorous. Corals are diverse, despite each colony originating from a single polyp.
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