In the visible region,the dispersive powers and the mean angular deviations for crown and flint glass prisms are $\omega, \omega'$ and $d, d'$ respectively. The condition for getting deviation without dispersion when the two prisms are combined is

• A
  $\sqrt{\omega d} + \sqrt{\omega' d'} = 0$
• B
  $\omega' d + \omega d' = 0$
• C
  $\omega d + \omega' d' = 0$
• D
  $(\omega d)^2 + (\omega' d')^2 = 0$