The Measurement of Cosmic Distances Was Helped Tremendously by the Discovery

Multiple Choice

The measurement of cosmic distances was helped tremendously by the discovery, in the early part of the 20th century, that in Cepheid variable stars, the average luminosity was related to:

A) their radial velocity
B) the abundance of hydrogen in their atmospheres
C) their distance from the Sun
D) the length of time they took to vary
E) their parallax