9.9 Given that $M$ is a positive number, the relationship between $\lg M$ and $\sqrt[10]{M}$ is
(A) $\lg M \geqslant \sqrt[10]{M}$.
(B) $\lg M \leqslant \sqrt[10]{M}$.
(C) $\lg M$ is never equal to $\sqrt[10]{M}$.
(D) None of the above.
(China Guangzhou, Wuhan, Fuzhou, and other five cities Junior High School Mathematics League, 1989)
$$
\begin{array}{l}
\text { [Solution] Since } \lg 10=1 \text{ and } \sqrt[10]{10^{5}}=\sqrt{10} \\
\lg 10^{10}=10=\sqrt[10]{10} \\
\end{array}
$$

Therefore, $(A),(B),(C)$ are all incorrect.
So the answer is $(D)$.