The slope of potential line at (x,y) is

Question

If the fluid velocity for a potential flow is given by V(x,y) = u(x,y)I + v(x,y)j with usual notations, then the slope of potential line at (x,y) is                                                                  

      (a) $\frac{V}{u}$                                (b) $-\frac{u}{v}$                          (c) $\frac{V^{2}}{u^{2}}$                               (d) $\frac{u}{v}$