What are the spiral arms of the milky way made of?

a. older stars and comets
b. variable stars
c. asteroids and comets
d. younger stars and gaseous nebulae

the spiral arms of the milky way was made of: D. younger stars and gaseous nebulae The spiral arms of the mily way was organized into four spiral arms, and since it was made of younger stars and gaseous nebulae, Spiral arms typically contain a higher density of interstellar gas and dust than the Galactic average as well as a greater concentration of star formation

The answer is b im not sure 

Answer the above statement is correct (true);