Which enzyme is required to prevent unwanted self-ligation of vector $DNA$ molecules in recombinant $DNA$ technology?

Two bacteria found to be very useful in genetic engineering experiments are

$A$ : $Pst I$ generates single-stranded overhanging stretches in $DNA$ after digestion that facilitate ligation.
$R$ : These short extensions can form hydrogen bonds with their complementary counterparts,which are then sealed by $DNA$ ligase to form phosphodiester bonds.

Nowadays, which of the following are known as "magic circles"?

Assertion : Plasmids are extrachromosomal $DNA$.
Reason : Plasmids are found in bacteria and are useful in genetic engineering.

If a foreign $DNA$ fragment is inserted into the $BamHI$ site of $pBR322$ to create $r-DNA$,in which medium will the recombinants not grow?