The divergence of a magnetic vector field $\vec B$ must be zero everywhere. Which of the following vector fields cannot be a magnetic vector field?

(a) $\vec B(x,y,z) = (y^2+z^2)\vec i + (x^2+y^2)\vec j - (x^2+2yz)\vec k$ ? could be cannot be a magnetic vector field.

(b) $\vec B(x,y,z) = -x\vec i + y\vec j - z\vec k$ ? could be cannot be a magnetic vector field.

(c) $\vec B(x,y,z) = (2xy - x^2 + y^2)\vec i + (y-y^2)\vec j$ ? could be cannot be a magnetic vector field.

(d) $\vec B(x,y,z) = (xy - xy^2)\vec i + (xy - x^2y)\vec j$ ? could be cannot be a magnetic vector field.