A two$-$dimensional incompressible steady flow field is defined by the velocity components as presented below. Knowing that a and b are constants and it satisfies the continuity equation, derive the stream function $(x,y)?$

$u=2a(\frac{x}{b}-\frac{y}{b})$

$v=-2a\frac{y}{b}$

a$.,(x,y)=2a(x\frac{y}{b}-\frac{y^2}{2}b)+c$

b$.,(x,y)=2ab(xy-\frac{y^2}{b})+c$

c$.,(x,y)=a(x\frac{y}{b}-\frac{y^2}{b})+c$

d$.(x,y)=2a(2x\frac{y}{b}-\frac{y^2}{2}b)+c$