Identify the set of points where the gradient ∇f\nabla f∇f of f(x,y)=x2+y2f(x, y) = x^2 + y^2f(x,y)=x2+y2 is the zero vector.
(0,0)(0, 0)(0,0)
(1,1)(1, 1)(1,1)
All points on the unit circle
The gradient is never zero