A trapezoidal channel is $10.0$ m wide at the base and has a side slope of $4$ horizontal to $3$ vertical. The bed slope is $0.002$. The channel is lined with smooth concrete (Manningâ€™s $n=0.012$). The hydraulic radius (in m) for a depth of flow of $3.0$ m is

- $20.0$
- $3.5$
- $3.0$
- $2.1$