$3.$ Classify the following functions as continuous $($specify the range$)$ or discrete:

$($i$)$ f $($x$)=$ ex

$($ii$)$ f $($x$)=$ axn$-1+$ b$($xo $-$ xn$)$ where xn represents a stage in a distillation column

$($iii$)$ f$($x$)=$ XD$-$Xs$1+$Xs where XD $=$ concentration of vapor from a still and XS$,$ is the concentration in the still?