For each function f(x, y,z), determine whether its gradient points radially outward from the origin, radially inward toward the origin, radially outward from the z-axis, or radially inward toward the z-axis. a. f(x, y, z) = 1/x^2+y^2
b. f(x, y, z) = x^2 +y^2
c. f(x, y, z) = 1/ x^2 +y^2 + z^2
d. f(x, y, z) = x^2 + y^2 + z^2