1. Given $I$ is the incenter of acute $\triangle A B C$, and $A_{1} 、 B_{1}$ 、 $C_{1}$ are the reflections of point $I$ over sides $B C 、 C A 、 A B$ respectively. If point $B$ lies on the circumcircle of $\triangle A_{1} B_{1} C_{1}$, then $\angle A B C$ equals $(\quad)$.
(A) $30^{\circ}$
(B) $45^{\circ}$
(C) $60^{\circ}$
(D) $90^{\circ}$
Prompt: From Conclusion 1, we know that $I$ is the circumcenter of $\triangle A_{1} B_{1} C_{1}$.
Answer: $\mathrm{C}$.