Why Is It Surprising to Find Spiral Arms in Spiral

Multiple Choice

Why is it surprising to find spiral arms in spiral galaxies?

A) Spiral arms require new stars, and previous new stars should have already used up the interstellar medium; no new stars should be forming now.
B) The variations in speed of the stars across galaxies are not high enough to twist the shape of the arms into a spiral.
C) There is no known mechanism to generate spiral arms.
D) The motions of the stars and differential rotation of the galaxy should have wound their arms tightly and made them blend and disappear.