Which of the Following Models Best Explains Why Our Galaxy

Which of the following models best explains why our galaxy has spiral arms?

A) The spiral arms are a wave of star formation caused by wave of density propagating outward through the disk of the galaxy.
B) The spiral arms are composed of groups of stars that are bound together by gravity and therefore always stay together as the galaxy rotates.
C) The spiral arms were imprinted on the galaxy at its birth. Ever since, like a coiling rope, the spiral arms have been wound tighter with each galactic rotation.
D) No model can explain the existence of the arms, which rotate with the galaxy like the fins of a giant pinwheel toy.

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