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

• A

  $\mathrm{r}={\mathrm{r}}_1+{\mathrm{r}}_2$

• B

  $\mathrm{r}={\mathrm{r}}_1^2+{\mathrm{r}}_2^2$

• C

  ${\mathrm{r}}^4={\mathrm{r}}_1^4+{\mathrm{r}}_2^4$

• D

  $\frac{1}{\mathrm{r}}=\frac{1}{{\mathrm{r}}_1}+\frac{1}{{\mathrm{r}}_2}$