Fibroblasts are cells that can be isolated from the connective tissue under the skin and grown in culture. Normally these cells retain the properties of fibroblasts as they divide for many generations in culture, but if we do an experiment in which the fibroblasts are forced to take up a highly expressed copy of the MyoD gene, they rapidly differentiate to become muscle cells.

a) MyoD expression is sufficient to cause fibroblasts to differentiate to muscle cells in culture
b) MyoD expression is correlated with muscle cell differentiation in culture
c) MyoD expression is necessary and sufficient to cause fibroblasts to differentiate to muscle cells in culture
d) MyoD expression is necessary for fibroblasts to differentiate to muscle cells in culture