$7.$ For an ideal stage in a distillation column, if the mole fraction of component A in the vapor is $0\mathrm{.}5$ and in the liquid is $0\mathrm{.}25,$ the relative volatility $\backslash (\backslash$alpha $\backslash )$ is $\_\_\_\_\_.$

$-$ a$)1$

$-$ b$)2$

$-$ c$)3$

$-$ d$)0\mathrm{.}5$