Describe the level curves of the function f(x,y) =

, where a > 0.
◦ all circles in the xy-plane
◦ the origin together with all circles in the xy-plane having centres at the origin and radii in the interval (0, a]
◦ all circles in the xy-plane having centres at the origin and radii less than a
◦ a circle in the xy -plane having its centre at the origin and radius = a
◦ all circles in the xy-plane having centres at the origin.