Answer:
b. genes have no difficulty crossing the species barrier.
Step-by-step explanation:
DNA is a universal molecule, by now it is known that has the same structure in pigs, birds, an other living things.
When a gene is transfer to other species, the most difficult action is that the living thing that is the receptor, can adopt this new sequence of DNA and reproduce it again as part of the new DNA.
In some examples like in pigs and humans, we are so closely that you can use some DNA pigs or tissues to replace in humans when is neccesary in a surgery.