# 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 of getting dispersion wih zero deviation, 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 ) $(\omega d)^2 + (\omega' d' )^2 =0$