Transplanting Animal Tissues and Organs into Humans Is Called Xenotransplantation

Transplanting animal tissues and organs into humans is called xenotransplantation. Which of the following is one difference that makes this process problematic?

A) Animal cells produce amino acids that humans do not produce.
B) Animals cells produce proteins foreign to human cells.
C) Human cells can be infected with bacteria found in animal cells.
D) Human cells and animal cells cannot be fused.
E) Animals cells produce sugar that human cells do not produce.