The stream function in $x-y$ plane for a flow is given by $=0\mathrm{.}5x^2{y}^{3.}$ The corresponding velocity

vector is

a$)x{y}^3$hat$()-1\mathrm{.}5x^2{y}^2$hat$()$

b$)1\mathrm{.}5x^2{y}^2$hat$()-x{y}^3$hat$()$

c$)1\mathrm{.}5x^2{y}^2$hat$()+x{y}^3$hat$()$

d$)x{y}^3$hat$()+1\mathrm{.}5x^2{y}^2$hat$()$