Here's why:

* Formation: Limestone is formed from the accumulation of the skeletons and shells of marine organisms like corals, mollusks, and plankton. These organisms contain calcium carbonate (CaCO3), which is the main component of limestone. Over time, these sediments are compacted and cemented together, creating the solid rock.

* Origin: It originates from the remains of living organisms, unlike igneous rocks (formed from molten magma) or metamorphic rocks (formed from existing rocks under intense heat and pressure).

Therefore, limestone is classified as a sedimentary rock, specifically a biochemical sedimentary rock because it is formed from the biological remains of organisms.