Type I diabetes is a form of diabetes that is due to the loss of insulin-producing cells of the pancreas. The potential of stem cells—in particular, induced pluripotent stem (iPS) cells—for therapy has gotten a lot of press. If researchers were attempting to stimulate the differentiation of iPS cells, which of the following would they most likely add to the cell-culture medium (the liquid surrounding the cells)? a. activin A, an extracellular signal protein b. Sox-2, a transcription factor active in early development c. Grb-2, an intracellular signal transduction protein d. lactase, an enzyme that catalyzes the breakdown of lactose