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 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.
B) Stars are spread almost uniformly throughout the disk of the galaxy outside the nuclear bulge,but the brightest stars occur only in the spiral arms,making the arms stand out.
C) There are many more stars in the arms than in the regions between the arms,so the arms stand out distinctly.
D) Stars occur only in the spiral arms (and the nuclear bulge) ,with essentially none between the arms,making the arms stand out brightly.