If \[_{i}{{\mu }_{j}}\] represents refractive index when a light ray goes from medium i to medium j, then the product \[_{2}{{\mu }_{1}}\times {{\,}_{3}}{{\mu }_{2}}\times {{\,}_{4}}{{\mu }_{3}}\] is equal to [CBSE PMT 1990]

• A.            \[_{3}{{\mu }_{1}}\]          
• B.            \[_{3}{{\mu }_{2}}\]
• C.            \[\frac{1}{_{1}{{\mu }_{4}}}\]                                           
• D.            \[_{4}{{\mu }_{2}}\]