Here's why:

* Sedimentary rocks are formed from the accumulation and cementation of sediments, which are particles of rock, mineral fragments, and organic matter.

* Limestone is primarily composed of calcium carbonate (CaCO3), which often originates from the shells and skeletons of marine organisms like corals, mollusks, and foraminifera.

* These organisms extract calcium carbonate from seawater and build their shells and skeletons. When they die, their remains accumulate on the seafloor.

* Over time, these remains are compressed and cemented together by dissolved minerals in the water, forming limestone.

So, limestone's formation through the accumulation and cementation of organic remains makes it a classic example of a sedimentary rock.