What Causes the Core of a Star to Contract During

What causes the core of a star to contract during the main-sequence phase of the star's life?

A) The conversion of hydrogen into helium reduces the number of particles in the core.
B) Convection in the outer layers carries energy out of the core more efficiently as the star ages.
C) Helium has a larger atomic weight than hydrogen and exerts a stronger gravitational pull on the core.
D) The rate of core hydrogen burning decreases as the hydrogen is used up, reducing the rate of energy generation.