Three thin prisms are combined as shown in the figure. The refractive indices of the crown glass for red,yellow,and violet rays are ${\mu _r}, {\mu _y}$ and ${\mu _v}$ respectively,and those for the flint glass are ${\mu _r}', {\mu _y}'$ and ${\mu _v}'$ respectively. If there is no net deviation in the yellow ray,then the ratio $A'/A$ will be:

• A
  $\frac{{2\left( {{\mu _v} - {\mu _r}} \right)}}{{\left( {{\mu _v}' - {\mu _r}'} \right)}}$
• B
  $\frac{{2\left( {{\mu _y} - 1} \right)}}{{\left( {{\mu _y}' - 1} \right)}}$
• C
  $\frac{{2\left( {{\mu _v}' - {\mu _r}'} \right)}}{{\left( {{\mu _v} - {\mu _r}} \right)}}$
• D
  $\frac{{2\left( {{\mu _y}' - 1} \right)}}{{\left( {{\mu _y} - 1} \right)}}$