What Is the "Faint Young Sun Paradox

What is the "faint young Sun paradox"?

A) By observing other stellar nebula similar to what astronomers believe our solar nebula looked like, it was determined that those stars were much brighter much earlier than predictions for the Sun.
B) Theories predict that the young Sun was faint. This would have allowed many comets to escape being vaporized at perihelion, and the impact rate on the Moon and similar bodies would have been much heavier than it was.
C) The Sun has become more luminous over time, but its temperature has remained relatively constant.
D) Earth has evidence of liquid surface water dating back 4.2 billion years, but the young Sun at that time was not luminous enough to have heated Earth sufficiently to prevent water from freezing.