Why do photosynthetic protists play a critical role in aquatic ecosystems?

A)Photosynthetic protists and prokaryotes are the major producers that form the critical base of food webs.
B)Photosynthetic dinoflagellates are in a mutualistic symbiotic association with coral, and coral reefs are one of the most diverse and productive biomes on earth.
C)They outcompete the non-photosynthetic parasitic protists.
D)They form colonies that cover the water's surface, which allows anaerobic organisms to thrive.
E)A and B