Which statement best describes the relationship between an object's density and its index of refraction?

a. as the mass per unit volume of the substance increases, the refraction index decreases.

b. as the optical density increases, the refraction index decreases.

c. as the physical density of the substance increases, the refraction index increases.

d. as the tendency of the atoms to absorb energy before reemitting it increases, the refraction index increases.