21. Acetyl-CoA plays a crucial role in the cellular metabolism. Which of the following best describes the primary function of acetyl-CoA? (A) To directly participate in the electron transport chain as an electron carrier. (B) To serve as a precursor for the synthesis of fatty acids, ketone bodies, and cholesterol. (C) To act as a structural component of the mitochondrial inner membrane. (D) To regulate gene expression by binding to specific DNA sequences. (E) To catalyze the hydrolysis of ATP for energy production.