The differential form of continuity equation for two dimensional flow of fluid may be written in the following form $\rho \frac{\partial \Psi }{\partial x}+\rho \frac{\partial \Psi }{\partial y}$ in which u and v are velocities in the x and y –direction and $\rho$ is the density. This is valid
(a) For compressible, steady flow
(b) For compressible unsteady flow
(c) For incompressible, unsteady flow
(d) For incompressible, steady flow