What Is a Probable Mechanism for the Initial Triggering, and the Subsequent

Multiple Choice

What is a probable mechanism for the initial triggering, and the subsequent maintenance, of the density wave that forms the visible arms in spiral galaxies?

A) equivalent of a massive "sound" wave generated at the center of the galaxy by the gravitational energy release at the initial formation of the galaxy, producing progressive compression of material that is shaped by galaxy rotation
B) gravitational influence from the rotating central supermassive black hole at the galactic center of the arm material
C) gravitational interaction from the close passage of another galaxy
D) shock waves from supernova explosions of massive stars within the spiral arms