One mole of ideal monoatomic gas $(\gamma = 5/30)$ is mixed with one mole of diatomic gas $(\gamma = 7/5)$. What is $\gamma$ for the mixture ? $\gamma$ denotes the ratio of specific heat at constant pressure, to that at constant volume.

Question

( A ) $4/3$
( B ) $3/2$
( C ) $23/15$
( D ) $35/23$