Why Do the Spiral Arms Show Up So Clearly in Spiral

Multiple Choice

Why do the spiral arms show up so clearly in spiral galaxies?

A) Stars are spread almost uniformly over the galaxy (outside the nuclear bulge) , but the brightest stars occur only in the spiral arms, where their brightness and the fact that they illuminate gas clouds make the arms stand out.
B) Stars are spread uniformly over the galaxy, but the dust forms a spiral pattern, absorbing starlight; the spiral arms are the dust-free regions between the dust lanes.
C) The number of stars in the arms is several times larger than in the regions between, so these arms are reasonably distinct.
D) Stars occur only in the spiral arms (and the nuclear bulge) , with essentially none between the arms, so the arms show up very distinctly.