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
Km is 
Select and explain. 
A. 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
B: Substrate concentration at which velocity of reaction is half of the maximum
Answer: B: Substrate concentration at which velocity of reaction is half of the maximum 
Explanation: Km is that concentration of substrate at which velocity of reaction is Vmax /2 (half of the maximum)
It corresponds to X-axis and denotes potency.