19.31 The number of non-acute angles in a convex 1992-gon is at least
(A) 1988.
(B) 1989.
(C) 1990.
(D) 1991.
(China Beijing Junior High School Mathematics Competition, 1992)
[Solution] The sum of the interior angles of a convex $n$-sided polygon is $(n-2) \cdot 180^{\circ}$, so the sum of its exterior angles is $2 \cdot 180^{\circ}$. Therefore, the number of obtuse exterior angles is at most 3, hence the number of non-acute interior angles is at least
$$
1992-3=1989 \text { . }
$$

Thus, the answer is $(B)$.