Astronomers Have Found the Existence of Spiral Arms in Galaxies

Multiple Choice

Astronomers have found the existence of spiral arms in galaxies difficult to account for because the

A) inner part of a galaxy rotates in a shorter time than the outer parts,so the arms should have wound up so tightly that they would have disappeared over the lifetime of the galaxy.
B) arms should have been destroyed by collisions with other galaxies over the galaxy's lifetime.
C) outer regions of a galaxy including the spiral arms have no significant rotation,so the arms should have fallen into the center of the galaxy.
D) outer parts of a galaxy rotate faster than the inner parts,so the arms should have straightened out into spokes like those of a bicycle wheel.