Here's why:

* Conglomerates are formed by the cementation of rounded gravel-sized fragments. These fragments, called clasts, are derived from pre-existing rocks (igneous, metamorphic, or even other sedimentary rocks).

* Sedimentary rocks are formed from the accumulation and cementation of sediments. Conglomerates fit this definition perfectly.

* Igneous rocks form from the cooling and solidification of molten rock.

* Metamorphic rocks are formed when existing rocks are transformed by heat, pressure, or chemical reactions.

Let me know if you'd like to learn more about sedimentary rocks!