What Useful Purpose Did RR Lyrae Stars Serve for Harlow

Multiple Choice

What useful purpose did RR Lyrae stars serve for Harlow Shapley in locating the galactic center?

A) Their brightness variations allowed accurate distances to be measured.
B) They emit copious amounts of infrared radiation and are thus visible through interstellar dust,which obscured visible light.
C) They are concentrated in the galactic center and so defined its direction.
D) They are important spiral arm tracers and thus defined the shape of the Galaxy.