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': 'Critical narrowing of coronary blood vessels is \nSelect and explain. \nA. 70%, B. 60%, C. 80%, D. 90%', 'role': 'user'}, {'content': 'Answer: A \nExplanation: None', 'role': 'assistant'}]