The reaction $\mathrm{S}\to \mathrm{P}$ is exothermic with $\Delta H=-30\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$. Its uncatalysed forward activation energy is $90\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$. An enzyme lowers the forward activation energy to $60\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$. What is the activation energy of the reverse reaction $\mathrm{P}\to \mathrm{S}$ in the presence of the enzyme?

1. A$120\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$
2. B$60\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$
3. C$90\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$
4. D$30\mathrm{kJ}\mathrm{mo}{\mathrm{l}}^{-1}$