15. As shown in Figure 3, the circumference of a circle with center $O$ is divided into 12 equal arcs, and the division points are labeled as $A, B, \cdots, L$. Then $\alpha+\beta=$ ( ).
(A) $75^{\circ}$
(B) $80^{\circ}$
(C) $90^{\circ}$
(D) $120^{\circ}$
(E) $150^{\circ}$
15. C.

Divide the circumference of $\odot O$ into 12 equal parts, with each arc having a degree measure of $\frac{360^{\circ}}{12}=30^{\circ}$.
Then $\alpha=\frac{1}{2} \angle G O E=30^{\circ}$,
$$
\beta=\frac{1}{2} \angle A O I=60^{\circ} \text {. }
$$

Thus, $\alpha+\beta=90^{\circ}$.