What Does Solving Equations on a Computer Have to Do

What does solving equations on a computer have to do with making a stellar model?

A) The equations apply the laws of stellar structure at locations within the star.
B) Equations can describe the H-R diagram and a star's location on it.
C) The mass-luminosity equation tells you how to find a star's luminosity given its mass.
D) Equations are used to model the nuclear reactions inside a star.