Which of the following sets of equations represent possible two dimensional incompressible flow cases?

(a) \(u=2 x y ; v=-x^{2} y\)

(b) \(u=y-x+x^{2} ; v=x+y-2 x y\)

(c) \(u=x^{2} t+2 y ; v=2 x-y t^{2}\)

(d) \(u=-x^{2}-y^{2}-x y t ; v=x^{2}+y^{2}+x y t\)