Cell differentiation is governed by genes. Depending on where in the body it is located, a given gene can be turned off, weakly on, or strongly on. For example, the gene for globin, which composes hemoglobin, is strongly on in cells that will mature into red blood cells and off in every other cell type.

Cells control gene expression by using molecular tags that lure an enzyme to insure the transcription of certain genes. They also use tags that repel the same enzyme for genes that are turned off. The enzyme that controls the transcription is

A) replicatase.
B) transcriptase.
C) DNA polymerase.
D) RNA polymerase.