In the same flow filed, What is the velocity at a point (2,1)?

Question

The stream function in a flow field is given by $\Psi$ = 2xy. In the same flow filed, What is the velocity at a point (2,1)?

(a) 4 unit                      (b) 5.4 unit

(c) 1.73 unit                 (d) 4.47 unit