In Chezy’s formula $${\text{V}} = {\text{C}}\sqrt {{\text{rs}}} $$   for calculating the velocity of flow in circular sewer of diameter running full, the value of hydraulic mean radius is

A. $${\text{D}}$$

B. $$\frac{{\text{D}}}{2}$$

C. $$\frac{{\text{D}}}{3}$$

D. $$\frac{{\text{D}}}{4}$$

E. $$\frac{{\text{D}}}{5}$$

Answer: Option D