Edwin Hubble Found the Great Nebula in Andromeda (M31 Galaxy)

Multiple Choice

Edwin Hubble found the Great Nebula in Andromeda (M31 galaxy) to be very far away by

A) comparing absolute to apparent magnitudes of Cepheid variable stars.
B) comparing absolute to apparent magnitudes of RR Lyrae variable stars.
C) measuring parallaxes of stars in M31.
D) getting radar echoes off stars in M31.