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: Pharmacology
[{'content': 'Km is \nSelect and explain. \nA. Substrate concentration at which the velocity of reaction is maximum, B. Substrate concentration at which velocity of reaction is half of the maximum, C. Corresponds to Y-axis, D. Denotes efficacy', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Km is that concentration of substrate at which velocity of reaction is Vmax /2 (half of the maximum)\nIt corresponds to X-axis and denotes potency.', 'role': 'assistant'}]