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: Physiology
[{'content': 'Vagal stimulation of heart causes \nSelect and explain. \nA. Increased heart rate, B. Increased R-R interval in ECG, C. Increased force of heart contraction, D. Increased cardiac output', 'role': 'user'}, {'content': 'Answer: B \nExplanation: None', 'role': 'assistant'}]