Using the Technique of Main-Sequence Fitting to Determine the Distance

Using the technique of main-sequence fitting to determine the distance to a star cluster requires that ________.

A) we have telescopes powerful enough to allow us to identify the spectral types of main-sequence stars of many masses in the cluster
B) the cluster be near enough for us to measure the parallax of its stars
C) we use ultraviolet and X-ray telescopes
D) we have a well-calibrated period-luminosity relation for Cepheid variable stars