Scientists have recently developed a process by which a skin cell from a human can be triggered to develop into a human heart muscle cell. This is possible because                   . a. most cells in the human body contain the genetic instructions for making all types of human cells; b. a skin cell is produced when all genes in the cell are expressed; turning off some genes in the cell results in a heart cell; c. scientists can add new genes to old cells to make them take different forms; d. a skin cell expresses only recessive alleles, so it can be triggered to produce dominant heart cell alleles; e. it is easy to mutate the genes in skin cells to produce the alleles required for other cell types