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: Medicine
[{'content': 'Wenckebachk phenomenon is defined as- \nSelect and explain. \nA. Progressive lengthening of PR interval till a beat is dropped, B. Slurred QRS complex, C. Irregular heart rate & permature ventricular beats, D. Shortened ST intervals', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]