Academic Decathlon Science Practice Test 2026 - Free Science Practice Questions and Study Guide

Movement in water is supported by fins, the specialized limbs of fish. Fins are flexible membranes reinforced by bony or cartilaginous supports that project from the body. They come in several types, each with a role: the tail fin provides forward thrust, pectoral and pelvic fins help with steering and braking, and dorsal and anal fins contribute to stability and prevent rolling. This description fits the idea that a fin is a limb used by fish for movement and stability in the water. The other options describe digestion, a brain region, or a protective shell, which aren’t related to fins.