Which language was used to compose texts like Rigveda and Upanishads?

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Here are verses from the Rigveda invoking Agni, the god of fire, often identified with the sacrificial fire, into which offerings were made so as to reach the other deities: "Bring, O strong one, this sacrifice of ours to the gods, O wise one, as a liberal giver. Bestow on us, O priest, abundant food. Agni, obtain, by sacrificing, mighty wealth for us." Verses such as these were composed in a special kind of Sanskrit, known as Vedic Sanskrit. They were taught orally to men belonging to priestly families.