At equilibrium, $K=k_f/k_r$ for $\mathrm{S}\rightleftharpoons \mathrm{P}$. An enzyme raises $k_f$ by a factor of $10^4$ at constant temperature. By what factor does it change $k_r$, and what is the new value of $K$ relative to the old value?

1. A$k_r$ is unchanged and $K$ increases by $10^4$
2. B$k_r$ decreases by $10^4$ and $K$ is unchanged
3. C$k_r$ increases by $10^4$ and $K$ increases by $10^8$
4. D$k_r$ increases by $10^4$ and $K$ is unchanged