A compound pipe of diameter d₁, d₂ and d₃ having lengths $${l_1},\,{l_2}$$  and $${l_3}$$ is to be replaced by an equivalent pipe of uniform diameter d and of the same length $$l$$ as that of the compound pipe. The size of the equivalent pipe is given by

A. $$\frac{l}{{{{\text{d}}^2}}} = \frac{{{l_1}}}{{{\text{d}}_1^2}} + \frac{{{l_2}}}{{{\text{d}}_2^2}} + \frac{{{l_3}}}{{{\text{d}}_3^2}}$$

B. $$\frac{l}{{{{\text{d}}^3}}} = \frac{{{l_1}}}{{{\text{d}}_1^3}} + \frac{{{l_2}}}{{{\text{d}}_2^3}} + \frac{{{l_3}}}{{{\text{d}}_3^3}}$$

C. $$\frac{l}{{{{\text{d}}^4}}} = \frac{{{l_1}}}{{{\text{d}}_1^4}} + \frac{{{l_2}}}{{{\text{d}}_2^4}} + \frac{{{l_3}}}{{{\text{d}}_3^4}}$$

D. $$\frac{l}{{{{\text{d}}^5}}} = \frac{{{l_1}}}{{{\text{d}}_1^5}} + \frac{{{l_2}}}{{{\text{d}}_2^5}} + \frac{{{l_3}}}{{{\text{d}}_3^5}}$$

Answer: Option D