What is the main function of the flashback in an occurrence at owl creek bridge

Read these lines from stanza 5 of the poem, “they shot wook kim” and answe the question. hung in the air/over that petroleum morgue that night. what does the use of the phrase “petroleum morgue” best suggest? an atmosphere of chemical detachment an unsettling place of ultimate rest a dark street containing a lifeless body an area containing putrid substances and smells