The primary purpose of the passage is to A. describe what results when a supernova triggers the creation of chains of star-forming regions B. propose a modification in the most widely accepted theory of spiral-galaxy structure C. compare and contrast the roles of clouds of gas and dust in two theories of spiral-galaxy structure D. describe a new theory of spiral-galaxy structure and contrast it with the most widely accepted theory E. describe a new theory of spiral-galaxy structure and discuss a reason why it is inferior to the most widely accepted theory