By mass, the interstellar medium in our region of the Milky Way consists of
A) 70% Hydrogen, 30% Helium.
B) 70% Hydrogen, 28% Helium, 2% heavier elements.
C) 70% Hydrogen, 20% Helium, 10% heavier elements.
D) 50% Hydrogen, 50% Helium.
E) 50% Hydrogen, 30% Helium, 20% heavier elements.