x - sin x 11. Let f(x) = if x * 0, Which of the following MUST be true? a if x = 0. (a) No matter what a is, f is discontinuous at x = 0. (b) f is continuous at x = 0 for some a but f is not differentiable there. (c) f is differentiable at x = 0 for some a and f'(0) = 1/6. (d) f is differentiable at x = 0 for some a and f'(0) = 1