How do you spell Arenae

Available Definitions:
1)  n. - The area in the central part of an amphitheater, in which the gladiators fought and other shows were exhibited; -- so called because it was covered with sand.
2)  n. - Any place of public contest or exertion; any sphere of action; as, the arenaof debate; the arena of life.
3)  n. - "Sand" or "gravel" in the kidneys.