Which Statement About Water Breathing in Fishes Is True

Which statement about water breathing in fishes is true?

A) Circulation of blood maximizes PO₂ on the internal side of the gill membrane.
B) Water flows unidirectionally in through the operculum, over the gills, and out through the mouth to the external environment.
C) Lamellae are thin to maximize diffusion distance between water and blood.
D) The one-way flow of water over the gills maximizes PO₂ on external gill surfaces.
E) Gill arches are the actual sites for gas exchange.