If $$l_1, l_2, l_3$$ etc. be the lengths and $$d_1, d_2, d_3$$ etc. be the diameters of the parts of a compound pipe, then length L and diameter D of a uniform equivalent pipe will be related as under

A

$$\frac{L}{D} = \frac{l_1}{d_1} + \frac{l_2}{d_2} + \frac{l_3}{d_3} + .....$$

B

$$\frac{L}{D^2} = \frac{l_1}{d_1^2} + \frac{l_2}{d_2^2} + \frac{l_3}{d_3^2} + .....$$

C

$$\frac{L}{D^3} = \frac{l_1}{d_1^3} + \frac{l_2}{d_2^3} + \frac{l_3}{d_3^3} + .....$$

D

$$\frac{L}{D^5} = \frac{l_1}{d_1^5} + \frac{l_2}{d_2^5} + \frac{l_3}{d_3^5} + .....$$