Determining which cases represent 2-D flow with constant density.

Which of the following sets of equations represent possible cases of two-dimensional flow with constant density?

a) u=x+y; v=x-y
b) u=x+2y; v=x^2-y^2
c) u=4x+y; v=x-(y^2)
d) u=x*t+2y; v=x^2-y*(t^2)
e) u=x*t^2 ; v=x*y*t+y^2