$6 \cdot 28 x$ is any real number, which of the following equations can be true?
(A) $\sqrt{x^{2}-1}=\sqrt{x+1} \cdot \sqrt{x-1}$.
(B) $10^{\lg x}=x$.
(C) $x^{0}=1$.
(D) $\sqrt[2 n+1]{x}=-\sqrt[2 n+1]{-x}$ ( $n$ is a natural number $)$.
(China Guangzhou, Wuhan, Fuzhou and other five cities junior high school mathematics competition, 1985)
[Solution] If $x<1$, then (A) does not hold;
If $x<0$, then (B) does not hold; if $x=0$, then (C) does not hold; while for any real number $x$, (D) holds.
Therefore, the correct choice is (D).