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 length of the shafts is

A

$$l_1 + l_2 \left( \frac {d_1} {d_2}\right)^4 + l_3\left( \frac {d_1} {d_3}\right)^4$$

B

$$l_1 + l_2 \left( \frac {d_1} {d_2}\right)^3 + l_3\left( \frac {d_1} {d_3}\right)^3$$

C

$$\frac{l_1 + l_2 + l_3}{3}$$

D

$$l_1 + l_2 + l_3$$