The Reason Most of the Terrestrial Planets Have Dense, Iron

Multiple Choice

The reason most of the terrestrial planets have dense, iron cores is because:

A) iron solidifies at the highest temperatures, so the iron core condensed first from the solar nebula. The rocky material then condensed directly onto the iron core as the nebula cooled.
B) terrestrial planets were initially molten or partially molten, and the iron sank to the center.
C) iron is magnetic, so there was a rapid accretion of iron dust grains into a core, followed by a slow accretion of rocky grains.
D) iron solidifies at the highest temperatures, so the iron core condensed first from the solar nebula. Rocky planetesimals then formed as the nebula cooled further, and gradually impacted onto the iron core.