$19 \cdot 17$ If the sum of the interior angles of a polygon is 3 times the sum of its exterior angles, then the number of sides of this polygon is
(A) 6.
(B) 7.
(C) 8.
(D) 9.
(1st "Five Sheep Cup" Junior High School Mathematics Competition, 1989)
[Solution] The sum of the interior angles of an $n$-sided polygon is $(n-2) \cdot 180^{\circ}$, and the sum of its exterior angles is
$$
n \cdot 180^{\circ}-(n-2) \cdot 180^{\circ}=360^{\circ} \text {. }
$$

According to the problem, we have $(n-2) \cdot 180^{\circ}=3 \cdot 360^{\circ}$, which gives $n=8$. Therefore, the answer is $(C)$.