$($D$)$ A furnace.

$9.$ Regarding the relative volatility $(,$ which of the following statement is true?

$($a$)$ An azeotropic mixture has an infinite relative volatility.

$($b$)$ It is defined as the ratio of the vapor pressure of two components.

$($c$)$ If the relative volatility of component A over component $B$ is $1\mathrm{.}02,$ it means that A and $B$ are theoretically separable based on their boiling point difference. However, it can be difficult.

$($d$)$ It has a unit of temperature.