Bacterial DNA labelled with ${}^{15}\mathrm{N}$ in both strands is allowed to replicate semi-conservatively for exactly two rounds in a medium containing only ${}^{14}\mathrm{N}$. The four resulting DNA duplexes are

1. Aall four with one ${}^{15}\mathrm{N}$ strand and one ${}^{14}\mathrm{N}$ strand
2. Bone with both strands ${}^{15}\mathrm{N}$ and three with both strands ${}^{14}\mathrm{N}$
3. Ctwo with one ${}^{15}\mathrm{N}$ strand and one ${}^{14}\mathrm{N}$ strand, and two with both strands ${}^{14}\mathrm{N}$
4. Dall four with both strands ${}^{14}\mathrm{N}$