How Did Henrietta Leavitt "Calibrate" Her Period-Luminosity Relationship for Cepheid

Multiple Choice

How did Henrietta Leavitt "calibrate" her period-luminosity relationship for Cepheid variable stars? In other words, how did she make the general idea into a numerical rule?

A) by finding cepheids in star clusters whose distance was known in another way
B) because the star closest to us is a Cepheid variable and we know its distance
C) by noting that the period was related to the luminosity in all stars
D) by measuring the Doppler shift in the spectral lines of Cepheids as they pulsated
E) by assuming that the Cepheids that appeared the brightest in the sky were closest to us
Section 19.4: The H-R Diagram and Cosmic Distances