9. (2.0分) A 66-year-old man with chronic hypertension is treated with prazosin by his physician. The treatment successfully decreases his blood pressure to within the normal range. What is the mechanism of the drug's action?A. Inhibition of β1receptors in the sinoatrial (S node)B. Inhibition of β1receptors in ventricular muscleC. Inhibition of α1receptors on vascular smooth muscleD. Stimulation of muscarinic receptors in the SA node