$Escherichia$ $coli$ $(E. coli)$ is widely used in biological research because:

$A$: $cDNA$ libraries are important to scientists in human genomics.
$R$: $cDNA$ is a synthetic type of $DNA$ generated from $mRNA$.

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A):$
Polymerase chain reaction is used in $DNA$ amplification.
Reason $(R):$
The ampicillin resistant gene is used as a selectable marker to check transformation.
In the light of the above statements,choose the correct answer from the options given below:

Which of the following bacteria is a close relative of $Salmonella$?

What do '$X$' and '$Y$' represent in the given diagram of $pBR322$?

$A$: Selection of recombinants due to inactivation of antibiotics is a cumbersome procedure.
$R$: It requires simultaneous plating on two plates having different antibiotics.