Let f(x, y) = 4 - √x2 + y2.
(A) Explain why f{a,b) = f{c,d) whenever (a, b) and (c, d) are points on the same circle with center at the origin in the xy plane.
(B) Describe the cross sections of the surface z = f(x,y) produced by cutting it with the planes x = 0, y = 0, and x = y.
(C) Describe the surface z = f (x, y).