T Cells Binding to MHC-Peptide Complexes Are Responsible for Acceptance

T cells binding to MHC-peptide complexes are responsible for acceptance or rejection of transplanted organs.To facilitate acceptance of the transplant, the ideal donor will have a close genetic relationship with the recipient -- even then, immune suppression drugs are often necessary.In a strange variation of this phenomenon, the endangered Tasmanian devil of Australia suffers from a devastating disease in which tumorous tissue is transmitted from animal to animal.What is the likely explanation for the lack of rejection of this transmissible tumor?

A) MHC genes are unique to humans.Without them, the T cells have no way to distinguish self and non-self.
B) Marsupials lack an adaptive immune system.
C) Low genetic diversity makes the MHC genes invariant, so the "transplanted" tissue is accepted as self.
D) Tumors are never targeted by T cells, unless they are caused by a viral infection.