19.15 If the number of sides of a convex polygon increases from 3 to $n$, then the sum of the exterior angles of the polygon
(A) increases.
(B) decreases.
(C) remains unchanged.
(D) cannot be predicted.
(E) becomes $(n-3)$ straight angles.
(1st American High School Mathematics Examination, 1950)
［Solution］The sum of the exterior angles of a convex $n$-sided polygon is:
$$
n \cdot 180^{\circ}-(n-2) \cdot 180^{\circ}=360^{\circ} \text {, }
$$

which is a constant.
Therefore, the answer is $(C)$.