A monoatomic 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 $\mathrm{V}$ to $8 \mathrm{~V}_{1}$ the efficiency of the engine is..
(A) $60 \%$
(B) $50 \%$
(C) $75 \%$
(D) $25 \%$