Starting from the Euler equation along streamline coordinate $s$: $\rho\partial v/\partial t + \rho v\partial v/\partial s = -\partial P/\partial s - \rho g\partial z/\partial s$. No viscous term is present (inviscid, confirms B). Integration is along $s$ only (confirms D). Dropping $\partial v/\partial t$ requires steady flow (so unsteady flow, C, breaks the form). $\int dP/\rho$ collapses to $P/\rho$ only if $\rho$ is constant (confirms A). \[ \boxed{A, B, D} \]