What Leads Astronomers to Believe That Many Large Moons Associated

What leads astronomers to believe that many large moons associated with the giant planets have compositions that are roughly half water?

A) Spectroscopic analysis indicates the presence of large bodies of water.
B) They have average densities midway between water and rock.
C) Space probes have drilled into the surfaces of many of the moons and detected water.
D) Rocks and other features that form only in the presence of water have been observed.
E) Astronomers have observed the gravitational effects of tides on those moons.