The thermal efficiency of a gas turbine cycle with regeneration in terms of $\mathrm{T}_{3}$ (maximum temperature), $\mathrm{T}_{1}$ (minimum temperature), $r_{p}$ (pressure ratio) and $\gamma\left(=\frac{c_{p}}{c_{v}}\right)$ is given by
(a) $1-\frac{\mathrm{T}_{1}}{\mathrm{~T}_{3}} r_{p}^{\left(\frac{\gamma}{\gamma-1}\right)}$
(b) $1-\frac{\mathrm{T}_{3}}{\mathrm{~T}_{1}} r_{p}^{\left(\frac{\gamma}{\gamma-1}\right)}$
(c) $1-\frac{\mathrm{T}_{3}}{\mathrm{~T}_{1}} r_{p}^{\left(\frac{\gamma-1}{\gamma}\right)}$
(d) $1-\frac{\mathrm{T}_{1}}{\mathrm{~T}_{3}} r_{p}^{\left(\frac{\gamma-1}{\gamma}\right)}$