The $...A...$ are the $DNA$ molecules that can carry a foreign $...B...$ segment into the host cell. Here $A$ and $B$ refer to:

Which enzyme is obtained from $Thermus\ aquaticus$ for the Polymerase Chain Reaction $(PCR)$?

Which organism's plasmid was used for the construction of the first $r-DNA$?

Consider the following statements:
$I.$ $A$ soil-inhabiting plant bacterium,$Agrobacterium \; tumefaciens$,a pathogen of several dicot plants,is able to transfer a piece of $DNA$ known as $T-DNA$.
$II.$ The $T-DNA$ causes tumours.
$III.$ Tumour formation is induced by $Ti-plasmid$.
Which of the statements given above are correct?

$pBR-322$ is frequently used as a cloning vector. What is it?

Plasmids are found in