Bacteria were grown in a medium containing a heavy isotope of nitrogen $(^{15}N)$ for many generations,and all their $DNA$ contained only heavy nitrogen. $A$ bacterium of this type was transferred to a normal medium and allowed to duplicate. After two divisions,the distribution of heavy $DNA$ is likely to be:

• A
  Only one daughter cell will have heavy $DNA$.
• B
  Two daughter cells have normal $DNA$ and the other two have both normal and heavy $DNA$.
• C
  All daughter cells have heavy $DNA$.
• D
  Half the daughter cells have heavy $DNA$ and the other half have normal $DNA$.