Consider a Cartesian coordinate system with orthogonal unit basis vectors $\hat{\imath}, \hat{\jmath}$ defined over a domain: $x, y \in[0,1]$. Choose the condition for which the divergence of the vector field $v=a x \hat{\imath}-b y \hat{\jmath}$ is zero.
- $a-b=0$
- $a < b$
- $a > b$
- $a+b=0$
