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
A 70-year-old woman had an ECG at her annual checkup. Using her lead II recording below calculate her hea rate 
Select and explain. 
A. 70, B. 92, C. 84, D. 104
A: 70
Answer: A: 70 
Explanation: 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)