"To come all that way, rifle in hand, with two thousand people marching at my heels, and then to trail feebly away, having done nothing—no, that was impossible."

What purpose does the end phrase separated by the em dash serve in this sentence?
Edit: The correct answer is "The phrase is meant to simplify and clarify the author's first thought." I'm 100% sure got full points for it on quiz.
For points just answer with "C. The phrase is meant to simplify and clarify the author's first thought."

However, the idea of real-life people living in what would become the united states for thousands of years before columbus ever left spain never dawned on me. what would be the best way to revise this sentence for greater clarity? a) however, the fact that columbus left spain thousands of years after people settled what would become the united states never dawned on me. b) it never dawned on me, however, the idea of real-life people living in what would become the united states for thousands of years before columbus ever left spain. c) however, the fact that people were already living in what would become the united states for thousands of years before columbus ever left spain never dawned on me. d) however, the idea never dawned on me that long before columbus ever left spain, real-life people lived for thousands of years in what would become the united states.

What is meant by “eternally burning black pipes”? what is this implying about the personality of the firemen?

Which sentence contains a restrictive