What Use Are 21- Cm Radio Waves to Galactic Astronomers

What use are 21- cm radio waves to galactic astronomers?

A) They bounce off stars like our Sun to let us precisely measure their distances.
B) They cut through the dusty cocoons to let us watch star birth.
C) They pick up the cool, dark matter much better than can optical telescopes.
D) Their Doppler shifts let us map the motions and locations of gas in the spiral arms.
E) We can reflect them off the core of the Galaxy.