Two heavy rotating masses are connected by shafts of lengths \[{{l}_{1}},{{l}_{2}}\] and \[{{l}_{3}}\] and the corresponding diameters are \[{{d}_{1}},{{d}_{2}}\] and \[{{d}_{3.}}\]this system is reduced to a torsionally equivalent system having uniform diameter \[''{{d}_{1}}''\] of the shaft. The equivalent length of the shaft is:

• A. \[\frac{{{l}_{1}}+{{l}_{2}}+{{l}_{3}}}{3}\]
• B. \[{{l}_{1}}+{{l}_{2}}{{\left( \frac{{{d}_{1}}}{{{d}_{2}}} \right)}^{3}}+{{l}_{3}}{{\left( \frac{{{d}_{1}}}{{{d}_{3}}} \right)}^{3}}\]
• C. \[{{l}_{1}}+{{l}_{2}}{{\left( \frac{{{d}_{1}}}{{{d}_{2}}} \right)}^{4}}+{{l}_{3}}{{\left( \frac{{{d}_{1}}}{{{d}_{3}}} \right)}^{4}}\]
• D. \[{{l}_{1}}+{{l}_{2}}+{{l}_{3}}\]