This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: ECG, Subject name: Physiology
[{'content': 'A 70-year-old woman had an ECG at her annual checkup. Using her lead II recording below calculate her hea rate \nSelect and explain. \nA. 70, B. 92, C. 84, D. 104', 'role': 'user'}, {'content': 'Answer: A \nExplanation: The hea rate can be calculated by 60 divided by the R-R interval, which is 0.86 second. This results in a hea rate of 70 beats/min. (1500/No of small squares between R-R interval - but R-R interval should be regular)', 'role': 'assistant'}]