The following four vector fields are given in Cartesian co-ordinate system. The vector field which does not satisfy the property of magnetic flux density is

- $y^2a_x+z^2a_y+x^2a_z$
- $y^2a_x+x^2a_y+z^2a_z$
- $x^2a_x+y^2a_y+z^2a_z$
- $y^2z^2a_x+x^2z^2a_y+x^2y^2a_z$