While Observing Your Goldfish, You Notice That Its Gill Flaps

While observing your goldfish, you notice that its gill flaps move up and down in a constant rhythm. This movement

A) helps the fish take in small gulps of air and store them in lung-like sacs.
B) requires less energy than moving air over the gills.
C) is unnecessary when the fish is swimming constantly.
D) ensures that the fish's gas exchange organs are surrounded by oxygen-rich water.