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Answer:
The air bladder which is also known as the swim bladder is an internal gas-filled organ that helps bony fishes to control their buoyancy thus, helping them to stay at their current water depth without any waste of energy in swimming.
A. Cartilaginous fishes swim constantly to avoid sinking to the bottom of the water column.
B. The tympanum is an external hearing structure in birds, some reptiles, some amphibians and some insects.
C. Cyclostomes are the jawless vertebrates which are devoid of scales, median fins are present which are supported by cartilaginous fin rays.
D, Bony fishes have four different kinds of scales: cosmoid, ganoid, cycloid, and ctenoid, whereas cartilaginous fishes have placoid scales.
So, the correct answer is cartilaginous fish have to swim constantly to avoid sinking.
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