The critical pressure ratio for maximum discharge in steam nozzle is

A

$$\frac{2}{\gamma + 1}$$

B

$$\left(\frac{2}{\gamma + 1}\right)^{\gamma}$$

C

$$\left(\frac{2}{\gamma + 1}\right)^{\gamma - 1}$$

D

$$\left(\frac{2}{\gamma + 1}\right)^{\frac{\gamma}{\gamma - 1}}$$