A compound pipe of diameter d1, d2 and d3 having lengths l1, l2 and l3 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. l/d² = (l₁/d₁²) + (l₂/d₂²) + (l₃/d₃²)

B. l/d³ = (l₁/d₁³) + (l₂/d₂³) + (l₃/d₃³)

C. l/d⁴ = (l₁/d₁⁴) + (l₂/d₂⁴) + (l₃/d₃⁴)

D. l/d⁵ = (l₁/d₁⁵) + (l₂/d₂⁵) + (l₃/d₃⁵)

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