If $f=$ moles of vapour present per mole of feed, then the slope of feed line is $($Mcabe$-$Thiele method$)$

$($A$)\frac{1-f}{f}$

$($B$)\frac{f-1}{f}$

$($C$)-\frac{1}{f}$

$($D$)-\frac{f}{1-f}$