Which of the following has popularized the $PCR$ (polymerase chain reaction)?

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?

The two bacteria found to be very useful in genetic engineering experiments are .............. .

Plasmids are used as vectors because ........

$DNA$ ligase is an enzyme that .......

Which structure is involved in genetic engineering?