1973 AHSME 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 \qquad \textbf{(B)}\ 4^6 \qquad \textbf{(C)}\ 4^8 \qquad \textbf{(D)}\ 4^9 \qquad \textbf{(E)}\ 4^{12}

Full credit goes to MAA for authoring these problems. These problems were taken on the AOPS website.