What is a plasmid?

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?

Identify the palindromic nucleotide sequence.

Which of the following key factors makes the plasmid a vector in genetic engineering?

Plasmids are suitable vectors for gene cloning because...

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