17.15 In $\triangle A B C$, the three sides $a, b, c$ are opposite to the interior angles $\angle A, \angle B, \angle C$, respectively. The exterior angle of $\triangle A B C$ at vertex $A$ is denoted as $\alpha$, and the exterior angle of $\triangle A B C$ at vertex $B$ is denoted as $\beta$. Among the following four statements, the correct one is
(A) $a=b+c$.
(B) $\angle B=\alpha$.
(C) $\alpha>120^{\circ}, \beta \geqslant 120^{\circ}, \angle \dot{C}<60^{\circ}$.
(D) $\angle B+\angle C=\alpha$.
[Solution] As shown in the figure, $a, b, c$ are the three sides of the triangle, so there should be $a\angle B$, thus option (B) is eliminated.
$$
\because \quad \alpha>120^{\circ}, \quad \therefore \quad \angle A<60^{\circ} \text {; }
$$

Also, $\beta \geqslant 120^{\circ}, \therefore \angle B \leqslant 60^{\circ}$.
If $\angle C<60^{\circ}$, then $\angle A+\angle B+\angle C<180^{\circ}$, which is a contradiction.
Therefore, option $(C)$ is eliminated.
Upon verification, the exterior angle of a triangle equals the sum of the two non-adjacent interior angles, so $\angle B+\angle C=\alpha$ holds true.
Thus, the correct choice is $(D)$.