A three-dimensional velocity field is given by $V=5x^2y \: i + Cy \: j-10xyz \: k$, where $i,j,k$ are the unit vectors in $x,y,z$ directions, respectively, describing a cartesian coordinate system. The coefficient $C$ is a constant. If $V$ describes an incompressible fluid flow, the value of $C$ is

1. $-1$
2. $0$
3. $1$
4. $5$