Select the correct inference of the given passage from "the cask of amontillado."

"the nitre! " i said; "see, it increases. it hangs like moss upon the vaults. we are below the river's bed. the drops of moisture trickle among the bones. come, we will go back ere it is too late. your cough--"
"it is nothing," he said; "let us go on.

a.) the narrator is worried about the nitre on the walls of the vault.
b.) the narrator knows it is already too late for fortunato to escape.
c.) (x) fortunato is worried about the moisture and bones in the vaults.
d.) fortunato wants to continue their descent into the vaults for the amontillado.