A foil is to be put as shield over a cake (circular) in a shape such that the heat is even along any diameter of the cake.Given that the heat on cake is proportional to the height of foil over cake, the shape of the foil is given by

• A. f(x, y) = sin(y/x)x2 + xy
• B. f(x, y) = x2 + y3
• C. f(x, y) = x2y2 + x3y3
• D. not possible by any analytical function