One of the Main Obstacles to Successful Cloning of Sheep

One of the main obstacles to successful cloning of sheep, apparently overcome by Wilmut et al., was:

A) to find an adult nucleus that was likely to work.
B) to synchronize the cell cycles of the donor cell and the egg cell.
C) to enucleate the donor cell.
D) the ability to culture cells in the laboratory.
E) to recognize that the nucleus from one sheep cell may contain different genetic information than the nucleus of another cell from the same animal.