Two heavy rotating masses are connected by shafts of length \(l_{1}, l_{2}\), and \(l_{3}\) and the corresponding diameters and \(d_{1}, d_{2}\), 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 equal to

(a) \(l_{1}+l_{2}+l_{3}\)

(b) \(\frac{l_{1}+l_{2}+l_{3}}{3}\)

(c) \(l_{1}+l_{2}\left(\frac{d_{1}}{d_{2}}\right)^{3}+l_{3}\left(\frac{d_{1}}{d_{3}}\right)^{3}\)

(d) \(l_{1}+l_{2}\left(\frac{d_{1}}{d_{2}}\right)^{4}+l_{3}\left(\frac{d_{1}}{d_{3}}\right)^{4}\).