A plane electro magnetic wave is propagating in free space. If average energy density at a point is u then amplitude of electric field is 

• A

  $\sqrt{\frac{2u}{{\in }_0}}$

• B

  $\sqrt{\raisebox{1ex}{$u$}\!\left/ \!\raisebox{-1ex}{${\in }_0$}\right.}$

• C

  $\sqrt{\raisebox{1ex}{$u$}\!\left/ \!\raisebox{-1ex}{$2{\in }_0$}\right.}$

• D

  $\frac{1}{2}\sqrt{\frac{u}{{\in }_0}}$