After purchasing a pot-bellied pig at the pet store in Springfield, Amy was informed by a Springfield city official that she would not be allowed to keep the pig as a pet, since city codes classify pigs as livestock, and individuals may not keep livestock in Springfield. The city official's argument depends on assuming which one of the following? ['Pigs are not classified as pets in Springfield.', 'Any animal not classified as livestock may be kept in Springfield.', 'Amy lives in Springfield.', 'Dogs and cats are not classified as livestock in Springfield.']