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

• A

  Only one daughter cell will have heavy $DNA$

• B

  Two daughter cells have normal $DNA$ and other two have both normal and heavy $DNA$

• C

  All daughter cells have heavy $DNA$

• D

  Half daughter cells have heavy $DNA$ and other half have normal $DNA$