Why Is Galileo Regio, the Large Circular Feature on Ganymede

Why is Galileo Regio, the large circular feature on Ganymede, so dark?

A) It is composed of dense basaltic mantle rock that once flowed like lunar mare.
B) It is the result of micrometeorite dust settling onto it over several billion years.
C) It has an asphalt-like material that is like the dark side of Iapetus of Saturn.
D) It is the result of ultraviolet light from the Sun interacting with the ice and creating asubstance that is quite dark.
E) It is the site of a recent meteorite impact that scattered a lot of dark, rocky material.