Consider the two-dimensional velocity field given by $\overrightarrow{V}=(5+a_{1}x+b_{1}y)\hat{i}+(4+a_{2}x+b_{2}y)\hat{j}$, where $a_{1}, b_{1}, a_{2}$ and $b_{2}$ are constants. Which one of the following conditions needs to be satisfied for the flow to be incompressible?

1. $a_{1}+b_{1}=0$
2. $a_{1}+b_{2}=0$
3. $a_{2}+b_{2}=0$
4. $a_{2}+b_{1}=0$