30-42 Let $p, q$ and $r$ be distinct primes, and we do not consider 1 to be a prime. Among the following numbers, the smallest positive perfect cube that contains the factor $n=p q^{2} r^{4}$ is
(A) $p^{8} q^{8} r^{8}$.
(B) $\left(p q^{2} r^{2}\right)^{3}$.
(C) $\left(p^{2} q^{2} r^{2}\right)^{3}$.
(D) $\left(p q r^{2}\right)^{3}$.
(E) $4 p^{3} q^{3} r^{3}$.
(36th American High School Mathematics Examination, 1985)
［Solution］Since $p^{8} q^{8} r^{8}$ is not a perfect cube, option $(A)$ is incorrect; also, since 4 is not a perfect cube, option $(E)$ is incorrect;
Meanwhile
$$
\begin{array}{c}
p^{3} q^{3} r^{6}<p^{3} q^{6} r^{6}<p^{6} q^{6} r^{6}, \\
\left(p q r^{2}\right)^{3}<\left(p q^{2} r^{2}\right)^{3}<\left(p^{2} q^{2} r^{2}\right)^{3},
\end{array}
$$

Therefore, the correct choice is $(D)$.