Form the PDE of the family of all the spheres whose centers lie on the xy-plane and have the constant radius r.
a) ∇²u = 0
b) ∂²u/∂x² + ∂²u/∂y² + ∂²u/∂z² = 0
c) ∂u/∂x + ∂u/∂y = 0
d) ∂u/∂x - ∂u/∂y = 0