In an Otto cycle, r is the compression ratio and $$ \gamma $$ is the adiabatic index.Which relation defines the air standard efficiency?

A

$$\eta = 1 - \frac{1}{r^{\gamma - 1}}$$

B

$$\eta = 1 - \frac{1}{r^\gamma}$$

C

$$\eta = 1 - \frac{1}{r^\gamma + 1}$$

D

$$\eta = 1 - \frac{1}{\frac{\gamma}{r^{\gamma + 1}}}$$