Spiral Density Waves Move Through a Medium That Is Also

Multiple Choice

Spiral density waves move through a medium that is also moving. The speed of the density waves

A) is greater than the speed of the medium through which they pass.
B) is the same as that of the medium through which they pass, thus forming a rigid spiral pattern.
C) is less than that of the medium through which they pass.
D) varies as the waves go around the galaxy, slowing down while passing through a spiral arm and speeding up in the regions in between.