"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."