Why Is 21-Cm Radiation a Good Tracer of Spiral Arm

Why is 21-cm radiation a good tracer of spiral arm structure?

A) Emitted by neutral hydrogen, it traces the location of high concentrations of gas.
B) Emitted by O and B stars, it traces the location of newly formed stars.
C) Emitted by ionized hydrogen, it traces the location of H II regions.
D) Emitted by molecular hydrogen, it traces the location of clouds in the process of collapsing into stars.
E) Emitted by red supergiants, it traces the presence of massive stars that are experiencing high rates of mass loss.

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