Why Are Low Doses of Antibiotics Commonly Used in Animals

Why are low doses of antibiotics commonly used in animals in industrial farming operations?

A) Killing some of the bacteria taking up nutrients in the guts of the animals leaves the animals more nutrients so they grow faster.
B) Low doses of antibiotics kill off any pathogens that might infect the animals, saving veterinary costs.
C) Any meat taken from these animals is essentially "purified" and pathogen-free.
D) They eliminate any pathogens that might be introduced by insects, common when a lot of animals are in one place.
E) People pay more for antibiotic-treated foods.