$28 \cdot 9$ If $a, b, c$ are all $n$-digit positive integers, then the number of digits in the positive integer $a b c$ is
(A) $3 n$.
(B) $3 n-1$.
(C) $3 n-2$.
(D) None of the above.
(2nd "Five Sheep Cup" Junior High School Mathematics Competition, 1990)
[Solution] $a b c$ could be a $3 n$-digit, or a $3 n-1$ or $3 n-2$-digit positive integer. For example, when $n=1$,
$$
1 \cdot 2 \cdot 3=6, \quad 2 \cdot 3 \cdot 4=24, \quad 4 \cdot 5 \cdot 6=120,
$$

which are $3 n-2, 3 n-1$, and $3 n$ digits, respectively. Therefore, the answer is $(D)$.