How Was Edwin Hubble Able to Use His Discovery of a Cepheid

Multiple Choice

How was Edwin Hubble able to use his discovery of a Cepheid in Andromeda to prove that the "spiral nebulae" were actually entire galaxies?

A) There are no Cepheids in the Milky Way,so his discovery proved that it had to be in another galaxy.
B) He measured the stellar parallax of the Cepheid in Andromeda,was able to determine the distance to it,and showed that it was far outside the Milky Way Galaxy.
C) From the period-luminosity relation for Cepheids,he was able to determine the distance to Andromeda and show that it was far outside the Milky Way Galaxy.
D) Since a Cepheid is a type of luminous galaxy,when he found it in Andromeda he was able to prove that Andromeda was a separate galaxy from the Milky Way.