Spectral Classification of a Star into the Lettered Categories, O

Spectral classification of a star into the lettered categories, O, B, A, F, G, K, and M is carried out by

A) finding the wavelength of peak emission in the continuum spectrum of the star.
B) determining the relative mass of the star by the study of binary star motions, in order to place it into its proper mass classification.
C) determining the total energy emitted at all wavelengths by the star, taking account of the full spread of wavelengths and their distances, in order to place the star into its luminosity class.
D) examining the relative amounts of absorption caused by various neutral and ionized atoms in a stellar spectrum.