Let $a$ and $b$ be real numbers. The function $f$ defined on the set of real numbers as
$$f(x) = x^{3} + 9x^{2} + ax + b$$
takes positive values on positive real numbers and negative values on negative real numbers.
What is the smallest integer value that $a$ can take?
A) 9
B) 13
C) 17
D) 21
E) 25