A thin prism having refracting angle $10^o$ is made of glass of refractive index $1.42$. This prism is combined with another thin prism of glass of refractive index $1.7$. This combination produces dispersion without deviation. The refracting angle of second prism should be :

Question

( A ) $4^o$
( B ) $8^o$
( C ) $6^o$
( D ) $10^o$