The sequence of the one-way gaseous exchange mechanism in a fish is
mouth → operculum → gills
mouth → gills → operculum
operculum → gills → mouth
gills → operculum → mouth
Correct answer is B
Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In some fish, capillary blood flows in the opposite direction to the water, causing counter-current exchange. The gills push the oxygen-poor water out through openings in the sides (opercula) of the pharynx.