$19 \cdot 28$ convex polygon, the interior angles less than $120^{\circ}$ cannot be more than
(A) 2.
(B) 3.
(C) 4.
(D) 5.
(China Zhejiang Ningbo Junior High School Mathematics Competition, 1987)
[Solution] The sum of the exterior angles of a convex polygon is $360^{\circ}$. If we set the number of its interior angles less than $120^{\circ}$ to $k$, then the corresponding exterior angles of these angles are greater than $60^{\circ}$.
But $k \cdot 60^{\circ}<360^{\circ}$, we get $k<6$.
Thus, the maximum value of $k$ is 5.
Therefore, the answer is $(D)$.