The Otto cycle thermal efficiency, with usual notation, is given as, where r is compression ratio and $$\gamma$$ is the adiabatic index

A

$$\eta_{o}=1-r^{\gamma-1}$$

B

$$\eta_{o}=1-r^{\gamma}$$

C

$$\eta_{o} = 1 - \left(\frac{1}{r^{\gamma - 1}}\right)$$

D

$$\eta_{o} = 1 - \left(\frac{1}{r^{\gamma}}\right)$$