$29 \cdot 38$ Let $m=n^{4}+x$, where $n$ is a natural number, and $x$ is a two-digit positive integer, then the value of $x$ that makes $m$ a composite number could be
(A) 16.
(B) 61.
(C) 81.
(D) 64.
(1st China Tianjin "Chinese Youth Cup" Mathematics Invitational, 1987)
[Solution] From $n^{4}+64=n^{4}+16 n^{2}+64-16 n^{2}$
$$
\begin{array}{l}
=\left(n^{2}+8\right)^{2}-(4 n)^{2} \\
=\left(n^{2}+4 n+8\right)\left(n^{2}-4 n+8\right),
\end{array}
$$

we know that when $x=64$, $m$ is a composite number.
Therefore, the answer is $(D)$.