Why Do We Suspect the Existence of Some Liquid Beneath

Why do we suspect the existence of some liquid beneath the surface of Callisto?

A) The interior appears to be highly differentiated, suggesting the existence of residual heat from the satellite's formation.
B) The satellite is close enough to Jupiter (which emits more energy than it receives from the Sun) so that its interior should be warm enough to keep pure water liquid.
C) The tidal churning caused by the pull of Jupiter should keep the interior warm enough to keep pure water liquid.
D) There is evidence of an induced magnetic field caused by Callisto's passage through Jupiter's magnetosphere.