Genetically modified (GM) crops are foods that have had their genomes modified, allowing humans to increase the production of certain crops. Farmer A purchases GM seeds that are immune to a specific herbicide. She plants these seeds in rows over seventeen acres. The following spring, Farmer A's neighbor, Farmer B, tells her that nearly a third of the plants in his crop fields have the same immunity. Which of the following describes the most likely explanation for this occurrence?

a. Rabbits which frequent both fields carried the seeds across to Farmer B's land.

b. Farmer A's crops cross-pollinated with Farmer B's crops on the neighboring land.

c. The two farms share an irrigation system which transmitted the immunity from one field to another.

d. By increasing the genetic diversity in Farmer A's crops, they have out-competed the crops on Farmer B's land.