In the late 1950s, Meselson and Stahl grew bacteria in a medium containing "heavy"
(radioactive) nitrogen (15N) and then transferred them to a medium containing 14N, which is a
nonradioactive form of nitrogen. They then extracted DNA from the bacteria and centrifuged it
to separate the DNA of different densities. Which of the results in figure above would be
expected after one round of DNA replication in the presence of 14N?
A) A
B) B
C) C
D) D