How Was Edwin Hubble Able to Use His Discovery of Cepheids

How was Edwin Hubble able to use his discovery of Cepheids in Andromeda to prove that the "spiral nebulae" were actually galaxies external to the Milky Way?

A) There are no Cepheids in the Milky Way, so his discovery proved that Andromeda had to be in another galaxy.
B) He measured the parallaxes of the Cepheids in Andromeda to determine their distances. He showed that Andromeda was far outside the Milky Way Galaxy.
C) He used main-sequence fitting to determine the distances to the Cepheids. He showed that Andromeda was far outside the Milky Way Galaxy.
D) From the period-luminosity relation for Cepheids, he was able to determine the distance to Andromeda. He showed that it was far outside the Milky Way Galaxy.
E) As a Cepheid is a type of luminous galaxy, Hubble's discovery of Cepheids in Andromeda proved that it was a separate galaxy from the Milky Way.