Radio Maps of Our Galaxy Show Spiral Arms Because the

Radio maps of our galaxy show spiral arms because the

A) arms have high Doppler shifts.
B) gas in the spiral arms is hot enough to emit photons.
C) dust in spiral arms is more dense than it is between the spiral arms.
D) gas in spiral arms is more dense than it is between the spiral arms.
E) stars in the spiral arms emit most of their energy at radio wavelengths.