That the narrator in edgar allan poe’s “the fall of the house of usher” finds himself becoming affected by usher’s condition is significant in what way?

a. it shows that the narrator is as frightened of madeline’s return as usher is.
b. it suggests that usher’s illness might have supernatural rather than genetic origins.
c. it demonstrates how contagious mental illness can be.
d. it proves that usher is actually pretending to be mentally ill.