This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pharmacology
[{'content': 'When a patient is already on atropine and i.v. administration of norepinephrine to him will lead to: \nSelect and explain. \nA. Decrease in heart rate, B. Increase in heart rate, C. Decrease in pupil size, D. Decrease in peripheral resistance', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]