The extent of our galaxy and our place within it was first found by Harlow Shapley with:
A) RR Lyrae variables in the globular clusters.
B) Planetary nebulae in the open clusters.
C) Radio emissions from Sagittarius A.
D) Infrared observations of the heat from its accretion disk around the black hole.
E) Cepheids of population I in the spiral arms.
