The Leading Explanation for the Existence of Spiral Arms Is

The leading explanation for the existence of spiral arms is:

A) shock waves from cyclic supernovae explosions.
B) shock waves from the quasar-like eruption of the Galactic core.
C) passages of spiral density waves through the interstellar medium.
D) passages of massive globular clusters through the Galactic plane.
E) tidal streams of small galaxies being torn up by our monster Galaxy.