For one dimensional flow of an incompressible fluid in unsteady state in x-direction, the continuity equation is given by

• A. ∂u/∂x = 0
• B. ∂(ρu)/∂x = 0
• C. ∂u/∂x = - ∂ρ/∂t
• D. ∂ρ/∂t = 0