If Young's modulus of iron is  $2\times {10}^{11}\mathrm{N}/{\mathrm{m}}^2$ and the interatomic spacing between two molecules is $3\times {10}^{-10}$ metre, the interatomic force constant  is

• A

  60 N/m

• B

  120 N/m

• C

  30 N/m

• D

  180 N/m