The Water in the Ocean Predicted to Occur Below the Surface

The water in the ocean predicted to occur below the surface of Jupiter's moon Callisto is heated above its freezing point by heat from

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