In the novel frankenstein, what is the significance of walton's letters to his sister at the beginning of the narrative?
a. walton's letters are full of descriptions of adventure and mystery and are an easy way to gain the reader's attention.
b. walton's letters contain an interesting background story and descriptions that are useful for the reader.
c. walton's letters to introduce the main characters of the novel, making it easy for people to understand them.
d. walton's letters speak of his thirst for knowledge and his loneliness, and they introduce the novel's main themes.