Which of the following enzymes is used in the multiplication (replication) of $DNA$?

$A$ $DNA$ molecule in which both strands have radioactive thymidine is allowed to duplicate in an environment containing non-radioactive thymidine. What will be the correct number of $DNA$ molecules that contain some radioactive thymidine after three duplications?

For how long should bacteria be cultured in a $^{14}N$ medium after being transferred from a $^{15}N$ medium so that the extracted $DNA$ consists of equal amounts of hybrid $DNA$ and light $DNA$?

The experimental system used in studies of the discovery of semi-conservative replication of $DNA$ was:

Given below are two statements.
Statement $I$ - The template strand with free $3'$ end is called leading template and the strand with free $5'$ end is called lagging template.
Statement $II$ - Replication always starts at the $3'$ end of the template strand and proceeds towards the $5'$ end of the template.
In the light of the above statements,select the correct option given below:

During $DNA$ replication,Okazaki fragments are used to elongate: