How many of the given statements $(i-iv)$ is/are correct?

$i.$ In transcription, adenosine pairs with uracil.

$ii.$ Regulation of lac operon by repressor is referred to as positive regulation.

$iii.$ The human genome has approximately $50,000\; genes.$

$iv. $ Haemophilia is a sex-linked recessive disease.

$A$ : Single $DNA$ dependent $RNA$ polymerase catalyses transcription of all types of $RNA$ in all except bacteria.
$R$ : Structural genes in bacteria are monocistronic.

Which of the following is required as inducer(s) for the expression of Lac operon?

A low level of expression of lac operon occurs at all the time. Can you explain the logic behind this phenomena.