For an incompressible flow field, $\overrightarrow{V}$, which one of the following conditions must be satisfied?
- $\bigtriangledown \cdot \overrightarrow{V}=0$
- $\bigtriangledown \times \overrightarrow{V}=0$
- $(\overrightarrow{V} \cdot \bigtriangledown) \overrightarrow{V}=0$
- $\frac{\partial \overrightarrow{V}}{\partial t}+(\overrightarrow{V} \cdot \bigtriangledown) \overrightarrow{V}=0$