AHSME 1983 Problem 12

If $\log_7 \Big(\log_3 (\log_2 x) \Big) = 0$, then $x^{-1/2}$ equals $\textbf{(A)} \ \frac{1}{3} \textbf{(B)} \ \frac{1}{2 \sqrt 3} \textbf{(C)}\ \frac{1}{3\sqrt 3} \textbf{(D)}\ \frac{1}{\sqrt{42}} \textbf{(E)}\ \text{none of these}$