15. The graph of f is given below. Identify the values of x at which f is not differentiable. Note: The graph of f has a vertical tangent at the point (0, 0). y X 3 [A] -2,2 [B] -1, 1 [C] -2, -1, 2 [D] -2, -1, 0,2 [E] -2, -1, 0, 1, 2