What Is the Faint Young Sun Paradox

What is the faint young Sun paradox?

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