Why Is 21-Cm Radiation a Good Spiral Arm Tracer

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

A) Emitted by neutral hydrogen,it traces the location of 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.

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