4 Classify each statement as true or false. Explain your answer. (a) If a graph is smooth at x = a, then it is differentiable at x = a. (b) If a graph is differentiable at x = a, then it is smooth at x = a. (c) If a graph is not differentiable at x = a, then there is a corner at x = a. (d) If a graph is horizontal at x = a, then it is not differentiable at x = a.