Which of these is a property of a harmonic function f(x,y)f(x, y)f(x,y)?
∇2f=0\nabla^2 f = 0∇2f=0
∇f=0\nabla f = 0∇f=0
∇×∇f=0\nabla \times \nabla f = 0∇×∇f=0
div f=1\text{div } f = 1div f=1