The hydraulic diameter, $$D_h$$, of a rectangular duct with sides 'a’ and 'b' is

A

$$D_h = \frac{4 ab}{2a + b}$$

B

$$D_h = \frac{2 ab}{a + b}$$

C

$$D_h = \frac{2 ab}{2a + b}$$

D

$$D_h = \frac{4 ab}{a + b}$$