Let $$f(x) = \begin{cases} x-1, & x \text{ is even} \\ 2x, & x \text{ is odd} \end{cases}$$, $$x \in N$$. If for some $$a \in N$$, $$f(f(f(a))) = 21$$, then $$\lim_{x \to a^-} \left\lfloor \frac{x^3}{a} \right\rfloor - \left\lfloor \frac{x}{a} \right\rfloor$$, where $$\lfloor t \rfloor$$ denotes the greatest integer less than or equal to $$t$$, is equal to:

A

121

B

144

C

169

D

225