<span>He it was who fought his way through the white water</span>
A. Readers gain insight into the narrator's belief that he is possessed by a demon.
This is the best option because the passage describes how he believes he has been possessed by a demon. From this passage, we don't actually know what happens once he is possessed. Option B says that it gives a reason for the narrator's behavior, but we don't know what that behavior is to make this a correct choice. Option C is wrong because first person invites us in to the narrator's mind. It doesn't not keep us at a distance. Option D is also not correct because we don't know anything about his treatment of a cat from this passage.
