In Meselson and Stahl experiment, E. coli was grown in a medium containing

• A

  ${ }^{40}\mathrm{KCl}$

• B

  ${ }^{24}\mathrm{NaCl}$

• C

  ${ }^{15}{\mathrm{NH}}_4\mathrm{Cl}$

• D

    $\mathrm{CsCl}$