The reaction pathway diagram shown below displays the energies of the reactants, products and the transition state in a reaction. Which expression shows the activation energy for the forward reaction?
- A$E_1 - E_2$
- B$E_1 - E_3$
- C$E_2 - E_3$
- D$(E_1 - E_2) - (E_2 - E_3)$