Most fishes do not sink in water because of the presences of: I. swim bladder II. air bladder III. air sacs IV. air in spongy bones

Analyze the question: It asks which structures help most fishes stay afloat (not sink) in water.

Explain the concept: Bony fishes (e.g., tilapia, catfish, carp) control their buoyancy with a swim bladder, a gas-filled organ in the body cavity. By adding or removing gas, the fish adjusts its density and floats at a particular depth without wasting energy. The swim bladder is also called the air bladder or gas bladder, so these are two names for the same organ.

Step-by-step evaluation:

• I. Swim bladder: Correct. This is the main buoyancy organ.
• II. Air bladder: Correct. It is another name for the swim bladder.
• III. Air sacs: Incorrect. Air sacs are found in birds (respiration and lightness for flight), not in fishes.
• IV. Air in spongy bones: Incorrect. Birds have pneumatic (air-filled) bones to reduce weight for flight. Fishes do not.

So only statements I and II apply to fishes.

Answer: Option B, "I and II are correct".

Tip: Link the structures to the animals: fish = swim/air bladder, birds = air sacs and hollow bones. If an option mentions air sacs or spongy bones in a question about fishes, eliminate it. (Note: cartilaginous fishes like sharks have no swim bladder. They rely on oily livers and swimming movement, but the question says "most fishes", which refers to bony fishes.)