Which of These Is MOST Directly Responsible for the Slow

Multiple Choice

Which of these is MOST directly responsible for the slow increase of the Sun's luminosity over time?

A) The increasing concentration of heavy elements in the Sun's convective zone.
B) The slow contraction of the Sun.
C) The gradual transformation of positively charged particles to neutral particles in the Sun's core.
D) The gradual increase of the Sun's magnetic field.