Concept:Fish stay buoyant in water mainly because of gas-filled organs called the swim bladder or air bladder.Explanation:The swim bladder and air bladder are the same gas-filled organ found in the body cavity of most bony fishes.This organ regulates buoyancy so the fish does not sink.Air sacs and air-filled spongy bones are not found in fishes for this purpose; they are features seen in birds.Therefore, only I and II are correct.Answer:B. I and II are correct.