The corresponding value of stream function is

Question

The velocity potential in a flow filed is $\phi$ = 2xy. The corresponding value of stream function is                 

(a)  (y² – x²) + constant

(b)  (x² – y²) + constant

(c) $\frac{1}{2}(x^{2}-y^{2}) + constant$

(d) 2(x-y) + constant