According to chapter 9 in fast food nation, how does manure get into the meat at slaughterhouses and hamburger grinders? a. manure is deliberately shoveled into piles of meat to increase the volume. b. the knives of slaughterhouse workers are not cleaned often enough. c. cooks fail to bring meat to a high enough temperature to kill pathogens. d. visitors often fail to clean their shoes, and they spread infectious agents on the floor.