Given below are two statements: 

Statement I : A semi-conservative method of DNA replication was experimentally proved by Messelson and Stahl. 
Statement II : The experimental proof was first shown in Escherichia coli using heavy isotopes of nitrogen and CsCl density gradient centrifugation technique

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

 DNA replicates semiconservatively.  It was shown first in Escherichia coli and subsequently in higher organisms. Matthew Meselson and Franklin Stahl performed the following experiment in 1958.

They grew E. coli in a medium containing 15NH4 Cl (15N is the heavy isotope of nitrogen) as the only nitrogen source for many generations. The result was that 15N was incorporated into newly synthesized DNA (as well as other nitrogen containing compounds). 

This heavy DNA molecule can be distinguished from normal DNA by centrifugation in a cesium chloride (CsCl₂) density gradient. 15N can be separated from 14N only based on density.

 Then they transferred the cells into a medium with normal 14NH₄ Cl and took samples at various definite time intervals as the cells multiplied, and extracted the DNA that remained as double-stranded helices. 

 The various samples were separated independently by CsCl₂ gradients to measure the densities of DNA.