A steady, two$-$dimensional, $2$ points

incompressible flow field in the

xy$-$plane has a stream function given

by

$=(-a{x}^3+b{y}^2+cy),$ where $a,b,$ and

are constants. The expression for the

velocity component $v$ is

a$.3a{x}^2$

b$.-3a{x}^2$

c$.2by+c$

d$.-2by-c$