Transplant of brain tissue relieves Parkinson-type symptoms in laboratory animals, but so far this procedure has shown little benefit for humans. What is one reason?

a. Most patients with Parkinson’s disease are unwilling to try this procedure.

b. Laboratory animals have different neurotransmitters than humans do.

c. Surgeons use this procedure only in patients with an advanced stage of the disease.

d. It is difficult to place the transplant into the correct location.