Interstellar gas is composed primarily of
a) 90% hydrogen, 9% helium, and 1% heavier elements.
b) molecules including water and CO2.
c) 50% hydrogen, 50% helium.
d) hydrogen, oxygen, and nitrogen.
e) 99% hydrogen, and 1% heavier elements.
Answer: a) 90% hydrogen, 9% helium, and 1% heavier elements.