In which one of the following sentences is but used as a preposition?

From this story and the indian saying which precedes it, what can the reader conclude about the culture in which the tale is set? a) they believe that death is always signified by pigeons flying away. b) they think that the behavior of creatures in the natural world can indicate things that will happen to people. c) they believe that honest, hard work provides relief from the ravages of the natural world. d) they think that death isn't the end, but only the beginning of a new life.