As the Early Sun Got Smaller in a Kelvin-Helmholtz Contraction

Multiple Choice

As the early Sun got smaller in a Kelvin-Helmholtz contraction, its interior became hotter and gained thermal energy. What was the source of this energy?

A) the tidal pull of a passing star
B) gravitational potential energy released by inward-falling material
C) nuclear energy from the interior
D) energy released by radioactive decay