AHSME 1973 Problem 19

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 $\textbf{(A)}\ 4^5 \textbf{(B)}\ 4^6 \textbf{(C)}\ 4^8 \textbf{(D)}\ 4^9 \textbf{(E)}\ 4^{12}$