The Most Striking Characteristic Observed on the Surface of Iapetus

The most striking characteristic observed on the surface of Iapetus, one of the outer satellites of Saturn, is:

A) a brightness variation, its leading hemisphere being as dark as asphalt whereas its trailing hemisphere appears as bright as ice.
B) plumes of nitrogen gas rising from cracks in the icy surface, apparently as a result of geyser activity.
C) a crater so large that the impact that created it must have come close to shattering the satellite.
D) an interconnecting network of parallel grooves, indicating tectonic activity in geologically recent times.