A butane isomerisation process produces $70$ k$.$mole$/$hr of pure iso$-$butane. A purge stream removed continuously, contains $85\%$ n$-$butane and $15\%$ impurity $($mole$\%).$ The feed stream is n$-$butane containing $1\%$ impurity $($mole$\%).$ The flow rate of the purge stream will be

$3$ kmole$/$hr

$4$ kmole$/$hr

$5$ kmole$/$hr

$6$ kmole$/$hr