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: Unknown
[{'content': 'Characteristic feature of the ECG is ? \nSelect and explain. \nA. Progressive lengthening of PR interval till a beat is dropped., B. Slurred QRS complex, C. Irregular heart rate and premature ventricular beats, D. Shortened ST interval', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Mobitz type I second-degree A V block (A V Wenckebach block) is characterized by progressive PR interval prolongation prior to block of an atrial impulse.', 'role': 'assistant'}]