13. A multicellular organism generally includes a wide variety of cell types. Examples in vertebrates include blood cells, neurons (cells of the nervous system), and the cells that line the digestive tract. What is the role of DNA replication and gene expression in cell differentiation?

A. Only the DNA that a cell will use is replicated, and all of the genes that it contains are always expressed.
B. Only the DNA that a cell will use is replicated, and gene expression is tightly controlled.
C. Nearly all DNA is replicated, and the omissions determine gene expression in the cell.
D. All DNA is replicated, but gene expression varies widely among cells and over time.