The electric field of a plane electromagnetic wave varies with time of amplitude $2{\mathrm{Vm}}^{-1}$  propagating along z-axis. The average energy density of the magnetic field is (in J ${\mathrm{m}}^{-3}$)

• A

  $13.29\times {10}^{-12}$

• B

  $8.86\times {10}^{-12}$

• C

  $17.72\times {10}^{-12}$

• D

  $4.43\times {10}^{-12}$