What is the criterion for the movement of $DNA$ fragments on an agarose gel during gel electrophoresis?

Biolistics (gene gun) is suitable for:

If a recombinant $DNA$ bearing a gene for resistance to Ampicillin is transferred into $E. coli$ cells,the host cells become transformed into Ampicillin-resistant cells. What happens when these $E. coli$ are grown on a medium containing Ampicillin?

Given below are two statements.
Statement $I$ - The isolated $DNA$ of donor organism is cleaved by suitable restriction endonuclease to form $DNA$ fragments with cohesive staggered or blunt ends.
Statement $II$ - The foreign $DNA$ or passenger $DNA$ can also be obtained directly from genomic or cDNA library.
In the light of above statements,select the correct option given below:

$PCR$ (Polymerase Chain Reaction) is used for the diagnosis of which of the following diseases?

In the context of blue-white screening,non-recombinant bacterial colonies appear blue in contrast to white recombinant colonies because: