One mole of ideal monoatomic gas $$(\gamma = 5/3)$$ 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.

A

$$3/2$$

B

$$23/15$$

C

$$35/23$$

D

$$4/3$$