Why Is 21-Cm Radiation the Best Way to Map the Spiral

Multiple Choice

Why is 21-cm radiation the best way to map the spiral arms in 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,penetrate the dense clouds of gas and dust in the disk.
C) This synchrotron 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.