In 1924, Edwin Hubble Proved That the Andromeda Galaxy Lay

In 1924, Edwin Hubble proved that the Andromeda Galaxy lay far beyond the bounds of the Milky Way, thus putting to rest the idea that it might have been a cloud within our own galaxy. How was he able to prove this?

A) By observing individual Cepheid variable stars in Andromeda and applying the period-luminosity relation
B) He was able to measure the parallax of the Andromeda Galaxy.
C) He found that the universe is expanding, and therefore concluded that Andromeda must lie outside our own galaxy.
D) He was the first person ever to look through a telescope at the object we now call the Andromeda Galaxy.