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 (Manning’s $n=0.012$). The hydraulic radius (in m) for a depth of flow of $3.0$ m is

1. $20.0$
2. $3.5$
3. $3.0$
4. $2.1$