You briefly expose bacteria undergoing DNA replication to radioactively labeled nucleotides. When you centrifuge the DNA isolated from the bacteria, the DNA separates into two classes. One class of labeled DNA includes very large molecules (thousands or even millions of nucleotides long), and the other includes short stretches of DNA (several hundred to a few thousand nucleotides in length). These two classes of DNA probably represent
A) Leading strands and Okazaki fragments.
B) Lagging strands and Okazaki fragments.
C) Okazaki fragments and RNA primers.
D) Leading strands and RNA primers.
E) RNA primers and mitochondrial DNA.

During DNA replication one strand is formed continuously and another strand is formed in a discontinuous manner. DNA polymerase can add nucleotides at 3' end only, therefore, the leading strand is formed continuously because it is synthesized in the direction of replication fork i.e 5' to 3' direction.

As lagging strand is synthesized in the backward direction they are synthesized in small pieces in a discontinuous way. These DNA fragments of the lagging strand are called Okazaki fragments. Then these fragments are sealed by an enzyme called DNA ligase.

So the two classes of DNA probably represent lagging and Okazaki fragments.