Scientists Studying the Runoff of Antibiotic Resistance Genes into Ground

Scientists studying the runoff of antibiotic resistance genes into ground water wells discovered in 2008 that concentrations of most resistance genes were diluted as water moved away from hog farms. One resistance gene, however, was found at higher concentrations in one well. Which of the following is a biologically valid explanation of this finding?

A) Exposure to antibiotics in their environment caused the bacteria naturally found in that well to mutate their DNA to create that antibiotic resistance gene.
B) The antibiotic resistance gene in the water was taken up by an organism living in that well. Since this gene helped the organism resist the antibiotics in the water, resistance was inherited by their offspring.
C) Hog gut bacteria that escaped into the water supply mated with other species of bacteria living in the well to create a "super-bacteria" containing the DNA of both species.
D) The herbicide resistance genes found in ground water well bacteria are easily converted to antibiotic resistance genes when an antibiotic is present in the environment.