Professor Marcus is studying gene expression in the liver cells of a mouse. She observes that when a steroid hormone enters the liver cells, the cells begin producing a wide variety of new proteins. Which of the following statements is the MOST USEFUL to include in an explanation of this observation? A. The steroid hormone contains several genes. B. A transcription factor activates a single gene that produces several proteins. C. A transcription factor activates many genes at the same time. D. The steroid hormone mimics the function of several mRNA molecules.