What Is the Physical Reason Astronomers Can Find the Luminosity

What is the physical reason astronomers can find the luminosity class (I, II, III, IV, or V) of a star using the star's spectrum?

A) The wavelength of maximum emission (given by Wien's law) is affected by the size of the star.
B) The relative amounts of hydrogen, helium, and other elements are different for stars of different luminosity classes.
C) The absorption lines in the spectrum are affected by the density and pressure of the star's atmosphere.
D) The absorption lines in the spectrum are affected by the star's surface temperature.