A human enzyme has maximum activity at about $37^{\circ }\mathrm{C}$. What happens to its activity when the temperature is raised well above the optimum?

1. AActivity keeps rising because the rate constant always increases with temperature.
2. BActivity falls because the enzyme is denatured and its active site is lost.
3. CActivity stays at its maximum value because the enzyme is a catalyst.
4. DActivity falls because the equilibrium constant of the reaction decreases to zero.