Why Is the Spectral Sequence of Stars Not Alphabetical

Why is the spectral sequence of stars not alphabetical?

A) The letters refer to the initials of the original discovers.
B) The original alphabetical labeling did not correspond to surface temperature and thus had to be reordered.
C) They were chosen to fit a mnemonic.
D) Because there is still uncertainty over what generates the energy in stellar cores.
E) Because it refers to stellar masses and these were difficult to measure accurately.