Reason : In between $A$ and $T$ there are three $H-$ bond, whereas in between $G$ and $C$ there are two $H-$ bonds.
$i.$ Glucose or galactose may bind with the repressor and inactivate it.
$ii.$ In the absence of lactose, the repressor binds with the operator region.
$iii.$ The z-gene codes for permease.
$iv.$ This was elucidated by Francois Jacob and Jacque Monod.