According to the nebular theory of solar system formation, why did Uranus and Neptune end up to be much less massive than Jupiter and Saturn?
a. Ices were able to condense at the distance of Jupiter and Saturn, but only rock and metal could condense at the distances of Uranus and Neptune.
b. The solar nebula was less dense where Uranus and Neptune formed, so accretion took longer, leaving less time to gather in nebular gas before the solar wind cleared it away.
c. The size differences are thought to be a random coincidence.
d. The colder gas in the outer regions of the solar nebula had less gravity and therefore could not gather up into such large balls as it could closer in.