The graph of z = f(x,y) is shown below. In each part, determine whether the given partial derivatives are positive, negative, or zero. (Note that the function is symmetric about 0 in both the x- and y- directions.) 505 0 -5 -10- y 20-2-4 (a) (-2,-2) and fxx(-2,-2) (b) (-2,-2) and (-2,-2) (c) (2.0) and fx (2.0) (d) (2.0) and yy (2,0)