Mice Whose Immune Systems Had Previously Rejected a Tissue Transplant

Mice whose immune systems had previously rejected a tissue transplant from a certain strain of donor mouse received another transplant shortly afterward, either from the same donor strain or a new one. All of the recipient mice eventually rejected the transplant. However, one group did so more quickly than the other group. Do you think this group received the graft from the original mouse strain (O) or the new strain (N)? Write down O or N as your answer.