Chapter 9, Problem 131P

Which choice is the differential , incompressible, two-dimensional continuity equation in Cartesian coordinates?
   ${\displaystyle \underset{CS}{\int }\rho \stackrel{\to }{V}\cdot \stackrel{\to }{n}dA=0}$

   $\frac{1}{r}\frac{\partial \left(rur\right)}{\partial r}+\frac{1}{r}\frac{\partial \left(u_g\right)}{\partial \theta }=0$

   $\stackrel{\to }{\nabla }\cdot \left(\rho \stackrel{\to }{V}\right)=0$

   $\stackrel{\to }{\nabla }\cdot \stackrel{\to }{\nabla }=0$

   $\frac{\partial u}{\partial x}+\frac{\partial u}{\partial y}=0$