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: Biochemistry
Activation energy is free energy difference between the ____: 
Select and explain. 
A. Substrate & product, B. Substrate & transition state, C. Transition & product, D. Sum of all the above
B: Substrate & transition state
Answer: B: Substrate & transition state 
Explanation: Activation energy is free energy difference between the substrate & transition state.
Activation energy may be also be defined as the minimum energy required to start a chemical reaction.
Reference-Harper 30th edition pg -75