Good Tracers of Spiral Arms in Galaxies Are

Good tracers of spiral arms in galaxies are:

A) red giants, because they are bright, and their long wavelengths can be seen through dust.
B) O and B stars, because they have short lifetimes and are thus always close to where they were formed.
C) red dwarfs, because they are the most numerous stars.
D) neutral hydrogen, because it can be observed via the 21-cm line.