One Reason That Cepheid Stars Are Important to Astronomers Is

One reason that Cepheid stars are important to astronomers is that

A) They are especially rich in metal content
B) There is a definite relationship between variability period and luminosity
C) They are all in the plane of the galaxy-so center can be measured
D) They can be seen even in distant clusters of galaxies
E) They are very young,so stellar evolution can be studied in detail