Based on the State$-$discharge relation below, it is TRUE to say that $($Select ALL that apply$)$:

a$.$ The stage$-$discharge relation offers a relationship to correlate water level in the river $($stage$)$ and discharges.

b$.$ The measurement of the discharge volume of the river is normally based on a direct measurement of flow.

c$.$ If the river is having a flow of $600cfs,$ we can expect the water level to be around $12ft.$

d$.$ When the water level in the river is at $8ft,$ we can estimate that the flow is around $1200cfs.$