Let f be a twice-differentiable function for all real numbers x. which of the following additional properties guarantees that f had a relative minimum at x=c?
(a) f’(c) = 0
(b) f’(c) = 0 and f”(c) < 0
(c) f’(c) = 0 and f”(c) > 0
(d) f’(x) > 0 for x < c and f’(x) < 0 for x > c

*the answer is not b*