19.32 Let $n$ be a natural number, and $n \geqslant 4$. Suppose the maximum number of acute angles in a convex $n$-gon is $M$, and the minimum number of acute angles is $m$; then the value of $M+m$ is
(A) 3.
(B) 4.
(C) A natural number greater than 4.
(D) Cannot be determined.
(Shanghai Junior High School Mathematics Competition, 1985)
[Sol] The sum of all exterior angles of a convex $n$-sided polygon is $360^{\circ}$. When $n \geqslant 4$, there are at most 3 obtuse exterior angles, meaning a convex $n$-sided polygon can have at most 3 acute interior angles, i.e., $M=3$.
A rectangle has four interior angles, each $90^{\circ}$, which are not acute, so $m=0$. Therefore,
$$
M+m=3 \text {. }
$$

Thus, the correct choice is $(A)$.