55. Two capillary tubes of the same length but different radii

r₁ and r₂ are fitted in parallel to the bottom of a vessel. The pressure head is P. What should be the radius of a single tube that can replace the two tubes so that the rate of flow is same as before

(a)  r₁ + r₂                                  (b)  r₁ ² + r₂ ²

(c)   r₁⁴ + r₂ ⁴                               (d)  None of these

Answer-d