For a convergent nozzle, what is the correct expression for the critical pressure, if the inlet pressure is $P_{1}$ ?
(a) $P_{1}\left ( \frac{2}{\gamma +1} \right )^{\gamma /\gamma ^{-1}}$
(b) $P_{1}\left ( \frac{\gamma +1}{2} \right )^{\gamma /\gamma ^{-1}}$
(c) $P_{1}\left ( \frac{2}{\gamma +1} \right )^{\gamma ^{-1}/\gamma }$
(d) $P_{1}\left ( \frac{\gamma +1}{2} \right )^{2\gamma /\gamma ^{-1}}$