Bacteria Were Grown for a Large Number of Generations in a Medium

Bacteria were grown for a large number of generations in a medium containing the nitrogen isotope ¹⁵N. The bacteria were then allowed to go through two cell divisions in a medium containing the normal (and lighter) isotope ¹⁴N, after which the DNA of the bacteria was extracted and analysed. This extracted DNA (X) was compared to DNA (Y) extracted from bacteria that had always been grown in a medium containing ¹⁴N. You would expect that

A) all X would be denser than Y.
B) the density of X would be the same as Y.
C) half of X would have the same density as Y and half of X would be denser than Y.
D) X would be less dense than Y.
E) one-quarter of X would be more dense than Y, and the remainder would be the same as Y.