2000 Fermat Problem 6

A student uses a calculator to find an answer but instead of pressing the \boxed{x^2} key presses the \boxed{\sqrt{x}} key by mistake. The student's answer was 9. What should the answer have been?

\textbf{(A)}\ 243\quad \textbf{(B)}\ 81\quad \textbf{(C)}\ 729\quad \textbf{(D)}\ 3\quad \textbf{(E)}\ 6561

If there are no answer choices shown, enter a numerical answer.

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