Which one of the following is satisfied if the flow is irrigational for a two – dimensional fluid element in the x-y plane?
(a) $\frac{\partial v}{\partial X}=\frac{\partial u}{\partial y}$
(b) $\frac{\partial v}{\partial X}= -\frac{\partial u}{\partial y}$
(c) $\frac{\partial u}{\partial X}= \frac{\partial v}{\partial y}$
(d)$\frac{\partial u}{\partial X}= -\frac{\partial v}{\partial y}$