A diatomic ideal gas is used in a Carnot engine as the working substance. If during the adiabatic expansion part of the cycle the volume of the gas increases from V to 32 V, the efficiency of the engine is
Since PVγ = constant
TVγ-1 = constant
TbVbγ-1 = TcVcγ-1
Tb/Vc = [Vc/Vb]γ-1 = [32V/V](7/5)-1
Tb = 4Tc = 4
i.e. 1 - (Tc/Tb) = 1 - (1/4) = 3/4
The correct option is B.