Which of the Following Is an Example of Gene Flow

Which of the following is an example of gene flow?

A) selective breeding of dairy cows to produce animals that can live in tropical climates and yield higher volumes of milk
B) the carriage of seeds between two separate urban forests by wildlife such as squirrels and rats that travel back and forth between the two
C) cross pollination by migrating honey bees between two geographically separate but genetically identical cotton fields
D) the passage of a recessive disease-causing allele from one generation to the next without the physical appearance of disease
E) the introduction of Burmese pythons,with no known predators or compatible mates,into the Florida Everglades