2. Dead Echinoderms are naturally recycled to make which product? They are the main ingredient in livestock feed. They are the main ingredient in toothpaste. They are the main ingredient in collagen. They are the main ingredient in limestone.

Echinoderms, such as sea stars and sea urchins, have hard, calcareous skeletons made of calcium carbonate. When they die, their skeletons settle on the ocean floor and contribute to the formation of limestone over time. Limestone is primarily composed of calcium carbonate, which is derived from the skeletal remains of marine organisms, including echinoderms. Therefore, the correct answer is that dead echinoderms are naturally recycled to make limestone.