"A nonagon is a nine-sided polygon." If this statement is true, then which of the following statements must also be true? A. A polygon is not a nonagon if and only if it has nine sides. B. A polygon is a nonagon if and only if it does not has nine sides. C. A polygon has 9 sides if and only if it is not a nonogon. D. A polygon is a nonagon if and only if it has nine sides.