Why Is It Surprising That Spiral Arms Are Stable Structures

Why is it surprising that spiral arms are stable structures, lasting billions of years?

A) Differential rotation should tear the arms apart.
B) Gas and dust clouds should run out of material to make young stars.
C) Supernova explosions should disrupt the arms.
D) Globular clusters are continuously passing through the arms.
E) Gravity should pull the arms down into the center of the galaxy.