Hydraulic mean depth $($Dm$)$ for a circular pipe of diameter $\text{'}$D$\text{'}$ flowing full is $0\mathrm{.}25$ D$.$ For a circular channel, at Dm $=0\mathrm{.}3$ D$,$ gives the condition for the maximum

Flow rate

Mean velocity

Both $\text{'}$a$\text{'}$ & $\text{'}$b$\text{'}$

Neither $\text{'}$a$\text{'}$ nor $\text{'}$b$\text{'}$