In Determining the Distance to a Galaxy by Using Observations

Multiple Choice

In determining the distance to a galaxy by using observations of a Cepheid variable star,which of the following is NOT needed?

A) The star's velocity via the Doppler effect
B) The Cepheid's period of variability
C) The type of spectrum of the Cepheid: metal-rich or metal-poor
D) The star's average brightness or apparent magnitude