Answer:
A. to suggest that people have both good and evil within them.
Explanation:
Gothic writers often use doppelgangers, or doubles, in their stories to explore the theme of duality, which suggests that people have both good and evil within them. By creating a character that is both similar and different to the protagonist, the writer can delve into the idea that individuals have both light and dark aspects to their personality. This can create a sense of unease and uncertainty in the reader, as they question which side of the character will ultimately prevail. Additionally, the use of doppelgangers can add an element of psychological depth to the story, as characters confront their own inner demons and struggles.