If $\log_7 \Big(\log_3 (\log_2 x) \Big) = 0$, then $x^{-1/2}$ equals
$\textbf{(A)} \ \frac{1}{3} \qquad  \textbf{(B)} \ \frac{1}{2 \sqrt 3} \qquad  \textbf{(C)}\ \frac{1}{3\sqrt 3}\qquad \textbf{(D)}\ \frac{1}{\sqrt{42}}\qquad \textbf{(E)}\ \text{none of these}$
Because $\log_7 \Big(\log_3 (\log_2 x) \Big) = 0$, we deduce $\log_3 (\log_2 x) =1$, and thus $\log_2 x=3$. Therefore, $x=8$, which means $x^{-1/2}=\frac{1}{2\sqrt{2}}$. Since this does not match any of the answer choices, the answer is $\fbox{{\bf(E)} \text{none of these}}$.