Which Statement About the Spiral Arms of Grand-Design Spirals Is

Which statement about the spiral arms of grand-design spirals is TRUE?

A) The density of matter in the spiral arms is many times the density of the regions in between.
B) The regions between the spiral arms are cloaked in dust which makes them look dimmer than the spiral arms.
C) the spiral density waves which move around the galaxy are transverse waves, moving up and down across the direction of the waves motion like light waves.
D) The spiral arms have almost the same density as the rest of the disk. They are more visible because they contain bright, newly formed stars.