Why Is the Innermost Galilean Moon Io Made Mostly of Rock

Why is the innermost Galilean moon Io made mostly of rock while the outermost Galilean moon, Callisto made of a mixture of ice and rock?

A) Io formed close to Jupiter where the temperatures were too high for ices to form
B) Callisto was captured as it wandered in from the Kuiper Belt
C) Io once had ice on its surface which was vaporized away by volcanism
D) Callisto experience many impact by icy comets which built up a layer of ice on its surface