Define $n_a!$ for $n$ and $a$ positive to be 
\[n_a ! = n (n-a)(n-2a)(n-3a)...(n-ka)\]
where $k$ is the greatest integer for which $n>ka$. Then the quotient $72_8!/18_2!$ is equal to 
$\textbf{(A)}\ 4^5 \qquad \textbf{(B)}\ 4^6 \qquad \textbf{(C)}\ 4^8 \qquad \textbf{(D)}\ 4^9 \qquad \textbf{(E)}\ 4^{12}$
Using the definition of $n_a!$, the quotient can be rewritten as
\[\frac{72 \cdot 64 \cdot 56 \cdots 8}{18 \cdot 16 \cdot 14 \cdots 2}\]
Note that for a given integer $x$, $\tfrac{72-8x}{18-2x} = 4$.  Since $0 \le x \le 8$, the quotient simplifies to $\boxed{\textbf{(D)}\ 4^9}$.