The Ocean Water That Is Believed to Occur Below the Surface

The ocean water that is believed to occur below the surface of Jupiter's moon Europa is kept unfrozen by heat from

A) absorption at this level in the moon of solar UV and visible radiation.
B) radioactive decay of heavy elements in the moon's core.
C) electric currents flowing in this conducting liquid, induced by the motion of the moon through Jupiter's magnetic field.
D) tidal distortions of the moon by the combined gravity of Jupiter and the other moons.