According to the Density-Wave Theory of Spiral Structure in Galaxies

According to the density-wave theory of spiral structure in galaxies:

A) collisions between galaxies create waves like the ripples around a rock dropped into a pond, and these are twisted into a spiral pattern by the galaxy's rotation.
B) collisions between stars send shock waves propagating through the disk of the galaxy.
C) star-formation regions propagate through the interstellar medium and get stretched into a spiral structure by differential rotation.
D) stars and interstellar clouds spend more time in regions of higher density, thus maintaining the higher density of the region.