Why Is 21-Cm Radiation One of the Best Ways to Map

Why is 21-cm radiation one of the best ways to map the spiral arms of the Milky Way?

A) The molecular hydrogen gas that produces this emission is concentrated in the spiral arms.
B) These photons, which are produced by neutral hydrogen, are not blocked by the dense clouds of gas and dust in the disk.
C) The emission is produced by supernovae, which are concentrated in the spiral arms.
D) Radio telescopes are easier to operate than optical telescopes, and observations can be made even during the daytime.
E) Radio photons do not have Doppler shifts, so we can detect clouds of gas without having to worry about the orbital velocities.