The length of the optical path of two media in contact of length \[{{d}_{1}}\] and \[{{d}_{2}}\] of refractive indices \[\]\[{{\mu }_{1}}\] and \[{{\mu }_{2}}\] respectively, is

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