If $w=f(z)=u(x,y)+i \: v(x,y)$ is an analytic function, then $\frac{dw}{dz}$ is:
- $\frac{ \partial u } {\partial x}- i \frac{ \partial u}{\partial y}$
- $\frac{ \partial u } {\partial x}+ i \frac{ \partial v}{\partial y}$
- $\frac{ \partial u } {\partial x}- i \frac{ \partial v}{\partial x}$
- $\frac{ \partial u } {\partial x}+ i \frac{ \partial u}{\partial y}$