Astronomers Now Understand That the Dark Regions or Rifts Visible

Astronomers now understand that the dark regions or rifts visible in parts of our Galaxy that are otherwise crowded with stars are caused by:

A) the absence of stars in regions that have been cleared out by recent explosions
B) starlight being blocked by otherwise invisible planets at the outskirts of our solar system
C) clouds with a considerable amount of dust which blocks the light of the stars behind them
D) huge groups of stars that only give off radiation in the infrared and not in visible light
E) holes in the structure of our Galaxy