Given below are two statements :

Statement $I:$

DNA polymerases catalyse polymerisation only in one direction, that is $5^{\prime} \rightarrow 3^{\prime}$

Statement $II:$

During replication of $DNA$, on one strand the replication is continuous while on other strand it is discontinuous.

In the light of the above statements, choose the correct answer from the options given below :

Semiconservative model of $DNA$ replication was proposed by which workers in eukaryotes

$DNA$ replication is aided by

Ten E.coli cells with ${ }^{15} N$ – dsDNA are incubated in medium containing ${ }^{14} N$ nucleotide. After 60 minutes, how many E.coli cells will have DNA totally free from $15\,N$ ?

Who used heavy nitrogen $N^{15}$ for proving semiconservative way of $DNA$ replication