Variable Stars,such as Cepheid Variables and RR Lyrae Stars,are Used

Multiple Choice

Variable stars,such as Cepheid variables and RR Lyrae stars,are used in what important measurement in astronomy?

A) Keeping of accurate time
B) Measurement of the gravitational fields of supermassive black holes by their effect on the star's pulsation rate
C) Measurement of the surface temperatures of stars
D) Measurement of distances to galaxies