The Structure of Ganymede, a Galilean Satellite of Jupiter, Is

The structure of Ganymede, a Galilean satellite of Jupiter, is thought to be

A) an iron core, a rocky mantle, and a liquid ocean with a dirty ice surface.
B) mostly or entirely rock, with no firm evidence of an iron core, and active volcanoes on its surface.
C) a large rocky core with a thin layer (100 km thick) of ice and water over it.
D) about half rock and half ice, with rock at the center and ice outside.