The Significant Feature of a Cepheid Variable Is That There

The significant feature of a Cepheid variable is that there is a relationship between two intrinsic parameters, one of which can be easily measured, while knowledge of the other parameter is required. These parameters are

A) period of brightness variation and spectral color.
B) variation of spectral color and distance to the star.
C) period of brightness variation and luminosity.
D) amplitude of brightness variation and luminosity.